I am going to third the opinion that the key is maintanance.
We had a 2003 Coachman Santara with about 25,000 miles on it...and it was as good as new when we "sold" it (read: traded it in). The biggest thing is regular, thorough maintanance.
Some things to think of (if buying from a dealership, negotiate these points):
tire condition
roof condition (get up there and LOOK at the roof)
awning condition
look for stains: ceiling, floor, by fridge, by toilet, by sinks, etc.
Does it LOOK like someone took care of it?
is the carpet in good condition?
negotiate a new mattress if you can
how is the battery (engine AND the two deep cycle batteries)
Smell for mold or musty - if there, it's hard to get rid of!
Check the air-vents and make sure the cranks work to open them.
Valve-stem extenders - if not equiped with this HAVE THEM ADD IT!
Others here have MUCH more experience than my husband and I, but these are some things we've learned along the way. We sure did love our Class C!