Every time we change oil in our class C they charge us extra

Servicing an RV requires either 1) a really large lift or 2) working on the ground. One requires extra capital costs and takes more valuable shop space. The other requires more time at each oil change. $20 seems very reasonable to me.

That said.... No all shops care about re-couping their extra costs since oil changes are almost always a loss-leader anyways. The real value is getting you in the door for other required services or repairs.