RVs typically have 5-15 miles per gallon. If you book a 32 feet class C RV then you are likely to get only 5-7 miles per gallon, smaller lighter Class C RVs may give you 10-12 miles per gallon depending on road conditions. Diesel based Class B RVs may give you more mileage. Always ask your RV Owner about the mileage and then reduce it by 10-20% for your budget planning.
If you are planning to make a 1000 mile round trip then at 10 miles per gallon you will need 100 gallons. At $2.5 per gallon you will spend $250 for gas. In another scenario if your rental RV provides you only 5 miles/gallon then you will spend $500 for your round trip.