Kirk brings up a good list of items, however be prepared for a lot of, I don't know answers. Today I went to look at a motorhome that was for sale by an individual on another forum (Trek Tracks, the Safari Trek owners forum) as a courtesy for a potential buyer that is outside the US as I was passing through the area.
The coach in question illustrates an all too common story out there for RV's on the used private sale market. The seller is a circa 65 year old man, who is selling the coach on behalf of his mother, it was really his father's coach, who passed away about a year ago, and no one knows anything about maintenance history, the coach has been sitting out in the back yard of his mother's house for a couple of years. The guy that is selling it is reasonably mechanically inclined, he just knows nothing about RV's, and lives some miles away from where it is located, while sitting the batteries ran down and died, tires dry rotted and have cracked, and various other things likely need attention (wiper blades, roof sealant, entry steps, etc., LPG/CO detector is alarming with end of life timer) The point being here is the seller does not know all the stuff that has went wrong with it while it has been sitting, just that his parents actively traveled in it until they didn't at some point I am guessing 2-4 years ago. By all appearances the coach seems to be a reasonably unmolested circa 14 year old coach, but I only gave it a once over for the potential buyer to let him know if there were any blatantly obvious show stopper issues, and what I found was a coach that appeared to have been well kept until a few years ago, when it was left to sit outside. No real show stoppers, just a lot of uncertainties that would require more extensive testing, connecting to shore power replacing batteries, etc. filling water and propane tanks, etc to determine, not to mention know knows what on the drive train that might only show up after many miles of driving. About all I can say for sure is that we were able to get the generator started thanks to a set of jumper cables, the microwave oven works, and the roof air conditioner does not (compressor starts, but fan does not blow).
The purchase of my current coach in 2016 is one owner removed from a similar situation, as the couple I bought it from, had bought it 2 years previously from the widow of its previous care giver, who again knew nothing of its maintenance history.