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

Look at your owners manual. It will tell the capacities, and compare.
That said, it is very possible that they jack the prices up for RV's just because. It's called perceived value, and what people are willing to spend for their toys. However, I would like to know if the oil capacities are the same, where on the bill is the added amount, or are they just putting it in a lump sum.