The first thing you learn about a classic "stick and tin" travel trailer is that they were built from the inside out, with every cabinet, seat, closet or bed forming an integral part of the support. Even the walls, which are only 3/4" thick wood framing with 1/8" plywood panels on the inside form a type of unibody construction, where everything separate is weak, floppy and insubstantial, but taken all together quite rigid.
Considering all that, the first order of business was removing the rear skin. The back corners had some water damage, so it was important to see what was hidden under the aluminum. What was hidden was....not great...as large portions of the rear framing and plywood fell apart into dust in our hands.
We also had some additional questions, as our floor structure was quite unique. Most Shastas and other trailers of this era were built with 3 inch floor joists and 5/8" plywood floors. From the outside, ours looked like 1 inch joists with 1/4" skins on the top and bottom...weird.