Good plan to have a quality independent shop do an inspection - regardless of whether you're buying from an individual or a dealer.
Individuals will lie just as much as dealers - don't assume otherwise.
Specifics to look into:
1. Check all house appliances (water heater, air conditioner, stove/oven, fridge (on propane and electric), furnace, water pump, generator (if applicable), converter/charger, inverter (if applicable), microwave), shore power connection, exterior of windows for gaps in trim/seals, interior of windows for signs of water intrusion, inside cupboards for signs of water intrusion, under cushions and mattresses for signs of water intrusions, outside seams for gaps/splits.
2. Engine - get it fully checked out.
3. Brakes - have them completely checked out even if told they were just replaced (mine HAD been just replaced but the idiots apparently forgot to put brake fluid in them - thank heavens my mechanic checked!)
4. Fuel lines and propane lines - check for any leaks. Better to check now instead of finding out on a camping trip that you have one.
5. Rubber - plan on replacing hoses, belts, and tires. They time out faster than they wear out on RVs.
Yes, you'll have maintenance costs - it's a given with an RV. How much depends on how carefully you check out before buying, how much you're willing to take on for the particular RV that you fall in love with, and how much you can do yourself. Be smart and even with a lot of work a used RV will cost a lot less than a new one.
If the seller won't let you test any item - NO MATTER THE REASON - DO NOT BUY IT. This is a huge red flag and so often you will hear a new owner say "well, I didn't test it because the seller didn't have..., couldn't connect..., just winterized..., etc., but he says it works just fine - and of course, once the buyer got it home and tested it, it did NOT work and cost $500+ to fix/replace.