I'm a small used RV dealer and drive many different RV's every month 450 miles back home. I plan for the worst and hope for the best but when your buying RV's with out the option to drive them first so times you get stung. I have most experence with gas powered 1989 - 1999 RV's It's great your doing this research and I hope my info can help.
Most to least common problems:
Tires - Used RV's tires are used and I have found over or under inflation as well as driving to fast to be the number 1 problem. Soulution buy new tires at least fronts/
Hanging brakes - Chevy bad brake hose or hoses. Ford sticking caliber
Ford fuel pumps on the fuel injected units mostly 1996 model RV's
Ford loss of power burnt up plugs or wires
1994 Fleetwood Flair sheered distributor pin ( I don't like Flairs)
No one likes to break down the stress level is off the chart and all at once. Just remember to stay calm relax and think. Most of the time what seems like a major problem is not and you can save time and money if you go at it alone.
Thats all for now tires and sticky brakes mostly. I'm just glad I drive them this far and deal with these common problems so my customers don't have to. There's a good tip when buying used ask the sales man, "How far did YOU drive it".