I don't know much about the 60s but am more familiar with the early 70's models. From what I think I know the model that was most prone to rear end sag was the early 70's Sovereign
(31") rear bath model. Several factors made this model prone to rear sag:1. longer rear overhang 2. extra weight in the rear from bathroom and water tanks. 3. rear floor rot from leaks in bathroom plumbing 4. lighter frame. The Airstream is a monococque design with the rear floor an integral part of the whole body's integrity. If the floor is rotten where it attaches to the frame and wall that's a problem. The recomended test procedure is to have a big strapping man, or woman, jump up and down with vigor on the back bumper. If the bumper moves but the coach body doesn't you got it.