How much should a pre-sale independent inspection cost?

Inspection services are not going to guarantee that the RV is free of problems. The seller may wish to guarantee or warrant the vehicle for a set amount of mileage or time but that is rare.
Observation or inspection done by a skilled third party has it's place but is not a guarantee against future problems. I'd suggest that anyone purchasing a used vehicle, RV or other, have a reasonable repair and maintenance budget in mind.