Bumping this to update any potential Airstream buyers of a few pitfalls I have learned about 1970's Airstreams:
They have wooden floors until like 2022 or so, which commonly rot because a leak can go unnoticed a long time due to the inner skin being aluminum as well. The leak goes down and gets the edge of the floor wet which is what ties the floor to the shell.
The all have rear end separation issues it seems because of the super common water leaks at the back access hatch. There was a 'fix' which involved two big bolts and washers back in the day, that masked the real issues. This can also cause the frame to rust.
The torsion axles are all junk by now and need swapped. Plan for $3000 for a pair of new ones. The rubber goes hard and starts to shake apart the trailer if you don't address it.
The frames are wet noodles and the body and frame depend on each other for strength, monocoque style. If the plywood gets bad at the edges and you loose this strength, they both start to sag especially on big ones like my 31.
So, I've stripped the bathroom out, removed the entire rear sheet of plywood, removed the C-channel that bonds the floor to the wall, replaced the rusted out sections of frame with some mobile home leftovers and angle iron, made a new floor section out of AC and covered the top and edges with thinned epoxy (neat stuff BTW), welded up a new T channel for attaching the floor to the body, and have it mostly bolted in. I had to spread the body and frame just enough to get the 3/4" sheet into place from the back.
I also bolted in some pieces of angle iron to tie the rear section of the frame to the axle support plates to make a pseudo truss to stiffen up the support of the rear floor. It worked way better than anticipated. Next step is to drill and rivet the floor channel to the body.