To add to Rich's comment, the Airstream is built so that the shell supports holds up the frame and the frame supports the shell. There's a certain degree of flex which is inevitable and planned for.
The rear joint (shell,floor, frame-no separation) must be intact, the axles fresh, and the running gear balanced or you'll end up with the dreaded sag.
What typically happens with these older units is the plywood floor rots at or near the rear joint creates "play" between the shell and the frame. With old, stiff axles and rough ride, the joint gets slapped back and forth and at some point, something gives. (Usually the frame, then the shell.)
Just make sure your rear joint is good and tight. (see pic)