It's rare that any type, weight or length of RV will get more than 10 MPG.
I used to worry about MPG but then decided the enjoyment of being out in nature far outweighed any associated costs.
You need to be prepared for possible repair costs since you will be buying an older used Class C. Problems begin appearing around the 5 year old mark and then really escalate after the 10 year old mark. Refrigerators, air conditioners, water pumps, toilets, batteries plus drive line components may fail. Most of these are $500 plus repairs.