Seeing as to how I own an older Class C I would offer this advice.
Check the oil. If it is pitch black walk away.
Check the transmission fluid. If it is brown, not pink, walk away.
Look under the engine. If it is covered with leaking fluids, walk away.
Take it for a drive:
Get it up to 50 MPH and downshift. If it doesn't slow down or sounds weird, walk away.
Take it up to 50 miles an hour and apply the brakes sufficiently enough to stop within 100 yards. If it shudders like the RV is falling apart, walk away.
Look at the coach. The condition of the coach will give you an idea of how much attention the previous owners gave the drive train.
I purchased my Class C two years ago with only 37K miles. Have not had any problems with the transmission or engine and it smog's to California standards so I would not necessarily eliminate a vehicle with low mileage. If the vehicle has 100K+ I would definitely have a compression test done on the engine.