It is likely that you will have to cut the corner that abuts the rear. Airstream trailers with twin beds in the rear have a custom mattress with a curved corner to fit.
Alternatively, so you know, AS did in fact make a "bunkhouse" model which is 30' in length (but actually 34' in exterior length measured from tongue to rear bumper) that was
specifically designed for families.
https://www.airstream.com/travel-tra...ans/30fb-bunk/
Notably, it features a dedicated front master bedroom and a double bed with bunk bedroom in the rear. AND, you can
still convert the couch and dinette into yet more additional bedding.
Square footage is one thing, but in actual daily use, how that space is arranged is so much more important. The stock floorpans of the 34 footers have much underutilized "wasted space" in the front living room.
The ideal solution might be to convert a 34 footer into a bunkhouse-like floor plan.