Help on Replacing Rear Panel on Shasta Compact

Welcome to the madness.

The real proper way to fix this damage is to start outside-in. Access to all the framing as well as the interior walls is made from the outside. The interior panels overlay the walls, and can't really be pieced in without losing the structural integrity of the overall build.

What you are seeing is typical to nearly all the Shasta models past 1968. The rear windows leak around them, as do the J-rails at the edges, after several years of putty drying out, and allows water to seep in around the framing and down through the rear wall. Water always runs downhill, so it rots from the bottom up. Usually the corners are where the damage starts, going up the wall corners and outward into the floor ply. The interior wall panels suffer de-lamination from the seepage.

My advice would be to do it once and do it right. If you are concerned about moving it, and are planning to replace framework and panels anyway, you can add cheater blocks to the inside to tie it together and shore up the framing strength. Adding t-post type supports from ceiling to floor will help support the weight of the roof structure and help to prevent collapse. This type of temporary support will be easy to remove when you get to the real repairs. Don't try to rush things, there are lots of resources to help understand the stuff you can't see. I used to have a load of blogs and photos of repairs from just this type of trailer but have lost most over the last couple years. I'll try to remember some of the ones I had and get you links to see them.
This is the rear of a '69 Compact. Don't feel like the Lone Ranger!