I used this service for an inspection on a used class C:
http://www.carzinspection.com/
I'm sure quality varies based on the individual performing the inspection, but they test drove it, went up on the roof, took a dozen or so pictures, noted potential water intrusions and damage, tested the appliances, and addressed specific questions I had about the unit that the seller couldn't answer. All for $250.
In my case I was buying a pretty cheap used unit sight unseen, and just wanted to make sure it wasn't a complete pile of garbage. The inspection served it's purpose, and I was quite happy with what I got.
The only thing they didn't catch was a vibration above 60mph that ended up being fixed by an alignment and/or new tires (not sure which).