When looking at used I would consider:
low mileage,
type of use (weekend, fulltime, long distance travel) both what previous owner used it for and how you plan to use it
overall condition- clean, shows little wear and tear...receipts for maintenance and repairs
be sure to check the roof and make sure it's in good shape no cracks and all seams are well sealed.
check ceiling and inside of cupboards plus the overhead bed/storage for any signs of leaks
test all of the appliances to make sure they work for more than just a short period...run the AC, fridge, generator and furnace long enough to make sure they don't have any issues.