I don't know if the access covers are available or not. Even if they weren't I certainly wouldn't use that as a basis for not buying an Argosy motorhome. I would bet that it can be repaired by a good skilled repairman. You would probably have to search out someone in the antique auto repair business. There are some amazing craftsmen out there.
I would start with a copy of Hemming's motor news and see what's available in the services section.
Does anyone know if any specific year(s) of Airstream trailers used this same filler?
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)