Firstly I would not take the salesman's word on anything unless he puts it in writing, has it notorized and warranties it :)
How do you know the previous owner started everything regularly?
My concern would be all of the plumbing - if it's gray PB then it's likely to split - if it's white PEX than that's a lot better. The waste tanks seals, the toilet seal. Check for any corrosion around the LPG tanks. I'd need to do a 100 mile trip (offer to pay for the gas) to confirm the engine and transmission was good and no seal leaks after it gets up to temperature and load.
Run the genny up to temperature and load it up with all 3 ACs and let it run for an hour.
You are putting close to $100k into a 15 year old unit that could have hidden issues that you have to get checked out before you purchase.