Wow... you're sure comfortable just wading in a slinging mud aren't you!
Actually there's a lot of MIS-information floating around about the '60s and '70s Airstreams... the first thing you need to understand is that all Airstream frames sag when unsupported by the shell. They're relatively light metal, and they're not built to be a solid platform frame. The unit is a true monocoque design (like an aircraft) and derives it's structural support from the body-frame combination, unlike stick built trailers that have a solid frame upon which a flimsy house is constructed. The frame's strength therefore doesn't come from the thickness of the steel as much as it does the design and how the body is supported by the number of 'outriggers' used over the length of the trailer, and how well the frame is supported by the shell.
Now, that said, during the Beatrice years and early Thor years, the interiors of the mid to late '70s used the then-space-age 'wood-look' vinyl contact papers... in some cases over really nice finished plywood veneers! At the time it was very modern and popular; with time though..actual wood veneer interiors have proven to be more popular, and the vinyl interiors have been judged to look "cheap". It's really a matter of preference; the vinyl clad interiors have actually held up pretty well. Another "space-age" better idea was changing from solid front cabinet doors to the roll-up tambour doors. They were pretty cool looking, but not terribly durable thirty years later.
Frame separation happened on some of the longer rear-bath models mostly in the early to mid-'70s trailers after larger tanks were added without reinforcing the shell/frame design. Most of those have now been retrofitted with repairs. The design was modified by the late '70s and those problems were resolved. You can do a search here on "frame separation" or "elephant ears" and find everything you'll need to know about that as well.
By '81 Airstream was nearly out of business... their production number were VERY low in the early '80s. You'll find that the '81 trailer you're looking at will be a very rare model indeed; but past the normal issues you look for in buying a thirty-year old Airstream, if the individual trailer is sound, it should be a very nice trailer indeed.