First and foremost I would give the rig a literal smell test. If it smells of dogs (or excessive air freshener) that would be enough for me to pass. It is much harder to replace the things that hold pet odors in an RV than it is in a stick home. Carpet, window covering, furniture etc. are all much more difficult to replace in an RV. Often the walls of an RV are of a construction that hold odors and they can't be painted over.
Next up would just be to look at the general wear and tear. It should be obvious what is OK and what is near the end of it's useful life.
As for the chassis, if they haven't moved it in 5 years, take that advice others have given and get it professionally inspected if everything else is to your liking. Lubrication of the axles, hubs and the like depend upon movement. Tires rot and many other items benefit from usage. This is a big enough purchase to get professional help.
As for pricing, it does take two to tango. Doesn't really matter what you think it is worth if the seller isn't willing to go that low. I would be more than willing to pay a premium price if the rig was what I wanted. Conversely, I wouldn't buy even a screaming bargain price if rig didn't fit my needs.
