Wow, it seems incredible that Airstream would not provide support for the shell and subfloor along the straight, rear section of the trailer. I imagine that area gets a lot of "accelerations", or forces from road bumps. Rear baths and their fixtures are relatively heavy. My trailer rear interior end cap is fiberglass, which is heavier than aluminum. My trailer has a 50 pound battery, a 10 pound converter, a black tank (which I moved forward), and a little storage. All this weight bouncing around like a big man on a diving board. I can't imagine Airstream making a trailer frame with no support for the subfloor and shell in that area.
My 66 Trade Wind has a structure angle welded there. But it is only about 1 & 1/2" piece and no more than 3/16 thick. I was happy it wasn't rusted away. It it was, I would have replaced it, no question.
KrisE is wise to have this important support added to her trailer. So is Dingo Girl.