A mid bath Sovereign
is a rare find. Congratulations. Mid bath is the most popular floor layout since the mid 80s.
I have a 75 Overlander 27' International rear bath. I have a friend who has a 75 Sovereign
rear bath 31' International. Both of our trailers have rear end separation and some frame droop. Both trailers have rotted subfloor at the rear of the trailer. Both of our trailers have rock hard axles.
I have made the necessary repairs to my Overlander last winter. I will finish some renovation work and give my trailer a good polish this winter. I have quite a bit to do.
My friend has this work to do on his trailer also, with time.
Maybe you have done the "bumper bounce" test on your trailer? Stand on the rear bumper over one of the frame rails and bounce up and down like you were on a diving board. Watch for any gap between the body and frame open up, or frame motion downward without the body moving with it. If this is observed, quite likely the rear of the body is no longer bolted to the frame due to a rotted subfloor and rusty rear cross member. Do the bounce test on the other rear frame rail also. Longer trailers like ours are more subject to this condition than the shorter Airstreams.
Also probe your subfloor for soft spots, especially around the entry door and under the windows. Some seventies trailers have trouble with the melamine laminate coming unstuck and pealing away. My Overlander is pretty solid in this regard, but I have seen other trailers with delamination on the cabinetry.
Old vintage Airstreams are indeed a lot of fun, but they often involve some pretty big projects. You can learn all about it here on these Air Forums. Here is a photo of my Overlander. And a link to my project thread if you are interested in my troubles.