Airstreams are build using semi-monoque constructions, which means (among other things) that the aluminum shell helps support the plywood floor and rather light and sparse frame rails. It is not build like a box sitting on top of a flatbed trailer. Instead, an Airstreams' frame and floor are weaker than a typical box trailer because they do not have to support the structure. Rather, the aluminum structure supports them. Cutting a door across the entire rear end will weaken the floor and frame, leaving it far too flexible and subject to damage.
Before proceeding with your project, ask yourself this: would you haul a RZR at 65-to-70 mph in the same position relative to the Airstream's tandem rear axles on a flatbed trailer having the same frame dimensions and frame components as your Airstream and with a 5/8" plywood floor? Seems to me the RZR might be hanging off a rather weak and flimsy rear end, causing bounce and excessive frame rail flex as you travel down the highway. Do new after-market RZR flatbed trailers use a 5/8" plywood floor? Or something stronger?
My 10' bed (16' overall length) tandem axle jeep hauling flatbed trailer has 2" deep x 10" wide by 10' long pressure treated boards bolted longitudinally across 2" x 4" rectangular steel tube cross members spaced on 2' centers. It's built for its job.
Finally, compare the depth and flange width of the frame rails on a new Airstream Eddie Bauer toy-hauler model with the narrower rear door than you need to the frame rails on your 1965
28' Ambassador. I believe you will find a substantial difference.
I vaguely recall someone else building a nice toy hauling Airstream. But I think he first fabricated a totally new frame with strong floor then mounted the Airstream body with a custom lift-up rear door on that. It was very expensive but very nice. Please do a search for that thread before proceeding.
I think your real challenge will be the steel and wood frame, not the aluminum body. So add your to your team your best steel and wood flatbed trailer friend too. I'd hate to see a Airstream Ambassador circumcised and then abandoned halfway through the procedure.