Like most trailers of this vintage, the walls in the old TMs were built with wood frames. No problem with that - unless the walls leaked and water got inside and rotted the wood. This happened only occasionally, but when it did, it was a serious problem, so you need to look for it.
Are there any water stains on the inside walls? If so, bad sign.
Run around the inside and the outside, and look at all the screws that are screwed into the walls. Tug on them. Do they pull out easily, or are they tight? If loose, the underlying wood may be rotted.
Several years ago, one of the board's members (he went by the name HappyTrails) rebuilt the walls in his TM to replace rotted wood. He did a very nice job, but it was a real challenge. He documented it all on his web site
http://members.aol.com/br768/
Other than that, the most important thing is probably to check all the appliances and make sure they work.
Bill