Quick Version:
-Install panelling
-Install 2x2 wall
-Install 2"thick rigid foam insulation
-Install layer of visqueen
-Install metal siding
Long Version:
In part 2 we left off with a replaced bathroom floor and the fixtures put back in. It was at this part of the process that I got pretty bad about remembering to take pictures, so there are not that many to help explain it all. I mentioned that as you remove the rotten members, we laid them out along an open area of ground so that we knew where they go for the rebuilding process. Essentially this was also our template for cutting new pieces. This is also the point where you wanna dig out that drawing of the open back wall with ALL the dimensions on it, horizontal AND vertical distances. You know...the one that I said was a crucial step to take PRIOR to tearing it apart that we did not do for ourselves?? Yeah...that crucial step...now it's time to see why it's so crucial.
Hubby put in the new piece of panelling before I could take a pic of what he did to prep it. He measured the bathroom window opening and lower storage opening then cut them out using a handheld jig saw (I believe, I'm not the best at tool names). He measured those areas based on the existing openings because the existing piece of ROTTEN panelling was removed in hundreds of pieces therefore could not be used as a template. We then measured every rotten 2x2 that we pulled out to cut a new 2x2 of the exact same length. These lengths CANNOT be too long nor too short, they MUST BE EXACT. Now, because we were measuring rotten pieces, I'm guessing that hubby cut them somewhere between an 1/8" to 1/4" too long...I'll explain later further down.
Once you have all the pieces cut, you can start assembling them back into the back wall. BE SURE to refer to your accurately dimensioned diagram and install the horizontal pieces as dimensioned!! We did not measure the vertical distances and were off in a couple places but did not learn this until I put the metal siding back on. We also cut all the 2" thick ridid foam insulation to fit into each spot. I actually just used the hand held jigsaw. You can see the areas where I had to piece together pieces because of the duct tape holding them together. We only bought one sheet of it. We also cut two new pieces of plywood supports for where the tail lights get screwed into. We got all these parts in then hubby had to work two days in a row. One day I yardsaled, the other I put on the visqueen and metal siding. We chose to put a layer of visqueen because of how the metal siding forms condensation on the interior. Originally, the pink insulation was in between all the wood and the siding creating somewhat of a vapor barrier. Since we went with a rigid insulation that fit in between 2x2's, those 2x2's would still have been exposed to condensation buildup on the aluminun siding. 
Hence a layer of visqueen, that's what we had. I cannot say it's the best option nor the worst. It's just what we had on hand (we're currently finishing our basement too) without having to make another purchase.I installed the visqueen layer without hubby's help. Lets just say a slight breeze does not help and makes for a longer session of visqueen installation. Oh, and it was all done with a weak, weak staple gun because I then had to go back and hammer every single one. Luckily I had a friend take pity on me and come over to help install the metal siding back on. Lets just say it was also a lesson in patience for her (yes she admitted that), because this was the part that we realized we were 1/8"-1/4" too long for the width of the wood and we realized (more importantly) that we did not have horizontal 2x2's to staple each piece of metal siding into!
THIS is why it is SO IMPORTANT to measure all those distances and write them down on a little drawing. Below is a pic of the siding as we were taking it off. You can see the bottom portion of it is folded into an "S" shape. The lower half of this "S" bend gets stapled back into the horizontal 2x2's (this was where we made a mistake.) The lower piece of siding fits up into this "S" bend. Once again, the purpose is to keep out moisture from the inside. Two whole rows did not have a single spot to staple into along the bottom edge, just on the two ends at the corner joints.
Once my friend and I got all those back metal siding's on, this is what we were left with for hubby's third day of work...in need of a window installation, a storage door installation and two corners to be re-puttied and screwed back in.
Well, I once again managed to NOT take pictures because we were down to the wire. Over the weekend, I went to an RV store and purchased three rolls of 3/16" thick putty tape. Putty tape is not easy to come by! I knew it was at the RV store just 20 minutes away, but shouldn't the hardware store(which is closer) carry this stuff too?? Apparently they don't. I was surprised at the number of people who have never even heard of the stuff!! I grew up knowing what this stuff was, for some reason I thought it was as common as duct tape. WRONG!
Well, I feel that this post is long enough. In part 4 I'll go into putty tape and our process for the corners. But before part 4 a quick tip.
Hubby put in the new piece of panelling before I could take a pic of what he did to prep it. He measured the bathroom window opening and lower storage opening then cut them out using a handheld jig saw (I believe, I'm not the best at tool names). He measured those areas based on the existing openings because the existing piece of ROTTEN panelling was removed in hundreds of pieces therefore could not be used as a template. We then measured every rotten 2x2 that we pulled out to cut a new 2x2 of the exact same length. These lengths CANNOT be too long nor too short, they MUST BE EXACT. Now, because we were measuring rotten pieces, I'm guessing that hubby cut them somewhere between an 1/8" to 1/4" too long...I'll explain later further down.Once you have all the pieces cut, you can start assembling them back into the back wall. BE SURE to refer to your accurately dimensioned diagram and install the horizontal pieces as dimensioned!! We did not measure the vertical distances and were off in a couple places but did not learn this until I put the metal siding back on. We also cut all the 2" thick ridid foam insulation to fit into each spot. I actually just used the hand held jigsaw. You can see the areas where I had to piece together pieces because of the duct tape holding them together. We only bought one sheet of it. We also cut two new pieces of plywood supports for where the tail lights get screwed into. We got all these parts in then hubby had to work two days in a row. One day I yardsaled, the other I put on the visqueen and metal siding. We chose to put a layer of visqueen because of how the metal siding forms condensation on the interior. Originally, the pink insulation was in between all the wood and the siding creating somewhat of a vapor barrier. Since we went with a rigid insulation that fit in between 2x2's, those 2x2's would still have been exposed to condensation buildup on the aluminun siding. Hence a layer of visqueen, that's what we had. I cannot say it's the best option nor the worst. It's just what we had on hand (we're currently finishing our basement too) without having to make another purchase.I installed the visqueen layer without hubby's help. Lets just say a slight breeze does not help and makes for a longer session of visqueen installation. Oh, and it was all done with a weak, weak staple gun because I then had to go back and hammer every single one. Luckily I had a friend take pity on me and come over to help install the metal siding back on. Lets just say it was also a lesson in patience for her (yes she admitted that), because this was the part that we realized we were 1/8"-1/4" too long for the width of the wood and we realized (more importantly) that we did not have horizontal 2x2's to staple each piece of metal siding into!THIS is why it is SO IMPORTANT to measure all those distances and write them down on a little drawing. Below is a pic of the siding as we were taking it off. You can see the bottom portion of it is folded into an "S" shape. The lower half of this "S" bend gets stapled back into the horizontal 2x2's (this was where we made a mistake.) The lower piece of siding fits up into this "S" bend. Once again, the purpose is to keep out moisture from the inside. Two whole rows did not have a single spot to staple into along the bottom edge, just on the two ends at the corner joints.Once my friend and I got all those back metal siding's on, this is what we were left with for hubby's third day of work...in need of a window installation, a storage door installation and two corners to be re-puttied and screwed back in.Well, I once again managed to NOT take pictures because we were down to the wire. Over the weekend, I went to an RV store and purchased three rolls of 3/16" thick putty tape. Putty tape is not easy to come by! I knew it was at the RV store just 20 minutes away, but shouldn't the hardware store(which is closer) carry this stuff too?? Apparently they don't. I was surprised at the number of people who have never even heard of the stuff!! I grew up knowing what this stuff was, for some reason I thought it was as common as duct tape. WRONG! Well, I feel that this post is long enough. In part 4 I'll go into putty tape and our process for the corners. But before part 4 a quick tip.TIP: DO NOT LEAVE PUTTY TAPE LAYING IN THE SUN NOR TRY TO INSTALL IT ON A SURFACE THAT HAS BEEN SITTING IN THE SUN.
Oh, the things we learn the hard way!
Happy Travels!
After tearing out all the 2x2's and panelling, we layed out the 2x2's on the driveway for a visual layout for when we're ready to build the wall again. IMPORTANT STEP: PRIOR TO REMOVAL OF 2x2'S, MEASURE VERTICAL DISTANCE BETWEEN HORIZONTAL MEMBERS AND WRITE IT DOWN ON PAPER. IN FACT, DRAW THE WHOLE WALL FRAME ON PAPER WITH EVERY MEASUREMENT. That's a huge step to remember to do, once you build the new frame, it all must be EXACT. You cannot build the framing a 1/4" longer or shorter, it screws up A LOT of other steps down the road...as you will soon see and learn from our mistakes (if not in this post, then in following ones)...
Once the whole back end is opened up, even more rot was exposed here (bottom left corner, that yellow is the original laminate flooring)...
...and here (bottom right corner).
I really, REALLY wanted to save the yellow laminate, but it was rotten too, I guess. Once we removed it, hubby just dug right in.
It literally just crumbled apart, so we pulled out the shop vac, it made for quicker removal.
See how much of that corner was rotten and is left?? Basically NOTHING!! The only thing holding it all together was the aluminum under-siding. And to think, I kept stepping on it to show people how soft the floor was. I'm lucky I didn't fall through!!
After clean up, hubby pulled out the circular saw and cut away to good stuff. Now it was time to rebuild. We purchased a dozen 2x2's, one 2x4, one sheet of 1/2" plywood, 2" thick rigid foam insulation, wood screws (all of which will be added to the sidebar showing our spending as 'Back End Remodel') and we had visqueen to use as a vapor barrier already. For some reason, I guess that single 2x2 on the end there was not rotton 'cuz it loos like hubby added new floor supports and attached them to it. There's also one piece of rigid insulation there on the left. Then next step was to measure and cut a new piece of plywood flooring, as shown in pic below.
Okay, so a little back story now...the reason we tore at the entire backend of the trailer in the first place was because we needed to install a new black water tank. Okay, you're thinking how does black water tank equal and back end tear out?? Well, hubby did not want to install the toilet back onto a 'soft' floor. We started by removing all the bathroom parts and pieces. Did you know that many parts and pieces of a travel trailer are installed BEFORE the front, back or even side walls are put in?? Yup, we learned this tidbit the hard way. We could not remove the bathroom vanity until we removed the shower liner, which both are in excellent shape so we wanted to keep them. Well, the bathroom liner was installed before the back end was sealed up because there is NO POSSIBLE way to get that liner in that tiny little door to the bathroom. So...that was our extended thinking of how black water tank replacement ended up being an entire back end remodel. Here's our new black water tank installed, which is a whole other blog post in itself...maybe later...on to remodel.
New plywood floor, shower liner back in, toilet back in,recycled peel-n-stick laminate flooring on new floor...now where is the vanity??
Ah, there, and I even didn't rotate the pic...
The next step after installing the majority of the bathroom components back is to start with the back end wall framing...but that will be in the next remodel post. 
With a 1PM departure and 6 hours (according to mapquest) of driving ahead of us, it was going to be a long day! We finally pulled into our campsite (above) around 9:30PM, just in time to make the beds and go to sleep. As we prepared for bed, we learned that we had absolutely NO POWER! My guess: the battery is dead. Hubby's guess: the 2yr old battery is not being charged by the truck. Either way, we had no power for bed time, just the propane. No power means no light and no water pump. That's okay, we packed bottled water and had full propane tanks.
The next morning we toured the Lewis & Clark Caverns. SO COOL!! Well worth a visit. In that pic above that tiny white column in the middle of the pic, that almost looks baseball bat-ish, is over 6 feet tall! There is no depth perception in a cave, when I took the pic, it looked as if it were a few steps away. Weird! We finally got on the road after lunch again, but this time we were only two hours north of West Yellowstone. Well...with a stop in Ennis, Montana to figure out the battery problem, we didn't arrive to our next campsite until 6pm. Whew! Another long day!
There's the culprit of this "woe" story. Did you know that most trucks are installed with a fuse for it's towing cable plug-in that it will blow if you're RV battery is wired BACKWARDS?!?!? Yes, I said that right...our RV battery, whoever installed it last (which for clarity sakes, WAS NOT us) wired it BACKWARDS! Now, if you're anything like me and not electrically savy...that's as far as the explanation will go. Here's a lengthier explanation as quoted from hubby, "the white wire and the black wire were wired so that the white wire was wired to the hot side when it should be the common." And if you need 
SHORT VERSION:
If you jumped the gun and ran out to your trailer, by now you're probably asking "How the heck do I remove those windows and access panels??" Well, some panel covers are simply screwed on and you can remove those screws and the panel. Others have hidden their screws in an ingenious U-channel to protect them from the elements and thus preventing rust and water seepage through the holes. Remember: ALL HOLES IN THE ALUMINUM SIDING ARE POTENTIAL WATER LEAKS! These U-channels have a plastic cover that slips into them, thus covering the head of the screw. One more measure taken to prevent leaks. 
Example of U-channel, windows left side with exposed screws, bottom still intack but I pushed the plastic cover over an inch.
This is an example where the plastic covers are mitered and meet in the corner. Not exactly sure how I will get these out without breaking them. At their age they are brittle and just chip out. Oh well....I can buy more of the plastic cover.
The tail lights and other exterior lighting is wired in and will need to be un-wired. I'm not the saviest electrician, but I'm pretty sure you should disconnect the battery FIRST before messing with the wiring. You could also cover the exposed wires with a wire nut, but being unplugged from the battery, there shouldn't be any current running through 'em.
This is the bottom front corner missing about 6" of corner moulding!! Not good. But it is a good close up of how the metal panels meet at the corner. Once you've located the corner moulding, our model of trailer has a flexible plastic cover that covers up the center strip where the screws are located. Remove this center cover to expose even more hex-head screws, remove the screws as far up as you need to go.
This pic shows the corner moulding and middle plastic cover separated from the trailer and unscrewed 3/4 of the way up the back. (As I type this post, I've realized that I've done a horrible job of taking pictures to show our process and have had to go out and take more.) Between the corner moulding and the aluminum siding there should be a layer of putty tape. A metal corner moulding sitting on metal siding does not seal out water, therfore putty tape is put in to help seal these corners. This putty tape will need to be scraped off, or in our case, chipped out because it was so old it lost it's puttiness. 
Where the two sides of aluminum siding meet, and beneath the putty tape will be a row of staples the entire length of the corner on both sides (and probably along the roof, we didn't go that far. This pic shows are front exposed corner which REALLY leaked at some point before we purchased the trailer.) These staples will need to be removed, and as rusty as they may be, make sure you have your tetanus shot and throw them away. We collected ours in an old Starbucks cup because we have two kids running around the area while we are working. This post sure is getting long winded....I'll add a quick review towards the top. 
Once the moulding is unscrewed and the staples are removed, the aluminum siding can begin to come off!! YAY! We're getting somewhere!! Our model had three different colors of siding,cream, lighter yellow and darker yellow. Each color is a different piece of aluminum, therefore we took off four different pieces of aluminum siding, two of which were cream colored.
The entire left cornere was crumbling apart rotton!! Which we suspected was in bad shape, but not crumbling. The main reason we opened this whole back side up was to replace the soggy bathroom floor (that little window is the bathroom window). So when we opened this up, we new we would have more to replace. Le sigh....oh well! We wanted a trailer to fix up a bit, I guess we got one. The next step is wood/paneling removal, floor removal and other little bits along the way, all coming up in the next post or two or three.