Here's another perspective. If the subfloor is totally rotten and the new flooring on top is the only reason you can walk from end to end, then sooner or later you will need to pull out the entire interior to replace the subfloor and inspect, repair, re-coat the exposed frame. Check the classifieds on this site. You can find trailers "ready to be fixed up" all day long for $5 - 8K.
If the subfloor is bad in spots, but not in imminent danger of crumbling, then you'll be able to use the trailer for a season or two, which has some value. We bought a 1964
fixer upper which we completely re-built before ever having gone camping in it. I would have done a few things differently if we had camped in the trailer first (but it was completely unlivable when we got it).
Polishing a trailer will cost $3 - 5K or a ridiculous number of hours + shoulder, wrist, leg pain + a few sunburns if you do it yourself. If the current polish is maintained, it will not be lost.
From the photos it looks like the upgrades are cosmetic which don't cost a lot, so unless they replaced appliances there hasn't been much real value added to the trailer.
So assume the value of the trailer is fixer-upper cost + polish job + whatever it's worth to you to have a "campable" trailer that you didn't have to drive to Montana/Florida/etc. to get.
When you go look at it you may be able to see what some of the subfloor looks like by opening the exterior hatches and looking in.