Class C under 30 ft. with gas engine

Routine service should be basically the same cost as servicing a van, since the chassis is the same (at least as far as things that need chassis service are concerned). The length of the vehicle really doesn't enter into that at all. It is true that most vans don't have duallies, which does add to some maintenance costs a bit (most obviously when buying tires). Length makes little if any difference in the costs on its own. I suppose a longer RV does have a bit more exhaust piping to replace when it wears out, but at that point it's splitting hairs.

In fact biggest single periodic expense is tires, which on most RVs need replacement due to age rather than treadwear. Last year it cost me just about $1000 for six new tires from a local store for comparatively inexpensive (i.e. not fancy name-brand) tires.