To try to answer your questions, there is no specific answer on mileage other than to say the average mileage for a motorhome is around 6,000 miles per year, the problem is without detailed logs it is hard to tell if a 10 year old 60,000 mile motorhome did 5,000-7,000 miles each year for 10 years, or if the owners did a 60,000 mile 24 month marathon circuit of the US then let it sit for 8 years. Also sitting itself is only part of the problem, lack of routine maintenance is the other rubber parts dry rot, some will argue this happens faster if they just sit around, but being used or not it happens. On daily driven car or truck these rubber components will often wear out before they age out, this is not true on an RV that gets comparatively few miles of use each year, in the end what this means is most rubber compounds are dry rotted to the point of crumbling after 10 or 12 years, this includes all sorts of hoses (radiator, fuel, oil and transmission cooler, brake, ..), various suspension and steering bushings (shocks, sway bar, tie rod, bump stops, etc.) and of course tires, which most people will suggest should be replaced every 7 years regardless of wear level (check the DOT code dates on the tires as a set of tires on a class A coach can run $3,000 - $5,000). RV appliances also tend to age out (air conditioners, water heaters, refrigerators) typically somewhere around the 15 year mark, though there are always those exceptions that seem to run forever. So when shopping for a used RV I would suggest buying one with a good maintenance history, where the previous owners have maintained and replaced parts as needed.
I bought my current coach out of state from a private seller 1,100 miles away in Florida, after being RV-less for about a decade, I am not saying what I did was right, I rolled a dice, and knew I was doing it. It was an educated dice roll though, I had been shopping for a coach for about 6 months, and had narrowed my selection down to a handful of models, and years. When this one popped up on a craigslist ad I emailed the seller, we exchanged a few emails, then talked on the phone for about an hour. The seller seemed knowledgeable about technical matters, common issues with this model, etc. (he was a retired electrician), I then did more online research, read the couple of dozen online messages he had posted over a couple of years on the brand owners group forum. I then went through the whole offer and counter offer process, explaining the reason for my offer, including links to similar models and explanation about added retrieval cost of buying his vs a similar unit that was several hundred miles closer. We ended up agreeing on a price about 25% below the asking price even though the ad had only been posted 2 days earlier. I would have preferred to have a professional inspection done, but the seller was in a rush to sell, so I skipped that did an over the phone hand shake (I offered to put down a deposit) got a plane ticket and flew down to buy it, pending it being as represented. The truth is there were a few things I felt were in worse shape than represented, though it would have taken more than that to make me walk away after having spent $500 on one way plane ticket, ...
Overall all things considered 2 years and 10,000 miles later I am happy with my decisions
