Hi chefmichael. Welcome to Air Forums! Judging from your post signature, you have aluminum-itis really, really bad! You have quite an inventory of old Airstreams. And I'll bet you know a lot about them.
There are some folks who like these late 40s Airstreams; Clippers and Liners and the like. I'll look up some of the treads. You might trying to look in the Airstream Knowledgebase toward the bottom of the Forums page. Then click on Liners, and then start reading.
Here is a thread I liked on a Liner. At least you can see what the structure looks like on these old Airstreams.
And Melody Ranch knows more about these old Airstreams than just about anybody. Read some of the rebuild threads form the "red barn". Fantastic work coming out of that barn.
It sounds like you need a frame repair, and you will likely need a new subfloor. These rebuilds are best done with the shell off. It will be much easier and much more efficient with the shell sitting in the yard while you repair away.
Airstream's pipe frame wasn't just the best idea they ever had. But it is light weight. Some folks have just built a new frame altogether. It depends on what your goal is with this antique Airstream.