From your description of the rear wall to roof molding being lifted, most time that points to roof sealant failures creating a leak either from the roof rubber to the rear top molding, a corner seal at the right rear corner or both. When the leak has been ongoing long enough, the rear rafter and wall sill plate rot out and the screws in the molding can no longer hold the molding to the roof. The aluminum molding can then lift up, allowing even more water to get in.
Water then pools in the ceiling and the walls and works its way down the wall. Water can also wick in from a seal failure of the putty tape on the corner molding allowing even more water into the wall/corner. The water keeps going down by gravity and the volume of water until it hits the black plastic bottom membrane where it cannot get out. It then festers and if the leak continues, the floor joists/floor starts to rot out.
Your plan of correction is in the right direction. Repair this damage from the outside working inwards. Do not start on the inside ripping the wall board out.
Both rear wall corners will need to be removed and then the rear wall siding. You can then see how far the water damage spread. You can fold back the roof rubber, but will need to remove both the left and right side gutter rails or at least unscrew then many feet down the wall to allow the gutter rail to hang out so the roof rubber can be lifted.
As to the side walls themselves, once you have the rear wall siding off and the roof opened up, you can see how much water infection is in the right side wall. If it comes to be that the right side wall siding has to come off, you can pull the entry door, cargo door and right side window out and remove the siding from the rear wall stopping at the entry door.
By this pic of the white wallboard being bubbled, I suspect you have enough water damage in the right side wall to require the siding to come off in that area, at least up to the entry door. You will know when you open it up.
This is all fixable if you have basic woodworking tools and an amount of time. It is going to take a good amount of time, if you have a helper, it does go faster. If you do not have an enclosed building to do the work in, using a tarp to cover the camper has been done before. I have had to use a tarp in the past for outside repairs and others have as well.
These 3 posts can help show you some how to take the camper apart and the repair. On the roof, you can do a repair to only the damaged section if that is the route you want to go. It will comes down to, how far did the water damage spread? You will know this when you open it up. I have other pics of doing a budboard repair and not the whole roof and can help with that when the time comes.
This is a complete front and rear wall, left side wall and total roof replacement project. It can help show one way how to do a repair like this, even though the floor plan is different. This is a slide camper, T264SR, yours is built very similar just no slide.
This post is a non slide camper, T1950 and I am working on it this winter. It is has a very similar issue as yours. I will keep posting as the repair continues over the winter.
Here is a 2004 T2475. It had a front wall issue that did get repaired. It also has a rear wall problem, but I did a patch on the rear wall roof area to buy me some time before I take it all apart for total restoration. This post is a step by step repair thread that I have not yet finished, but is a good start and may help.
Other club members have done similar repairs and have made posts as well. We have a lot of help here on Sunline Club, all you need to do is ask and we will help as we can.
Hope this helps