Expect to get 7-9 mpg and be pleasantly surprised if you ever get 10mpg on flat lands with tail winds at 55mph. RV's are heavy and have a lot of rolling resistance and are not streamlined. Have tires inflated to proper pressure, usually 65-70 psi in front and 75-80 psi in rear duals. Have tires inspected and make sure that your tires are less than five years old by date codes on each tire and that they don't have sidewall cracks. A tire coming apart or blowout can result in loss of control and damage to things under the RV due to whipping steel belt. Have brakes and engine cooling system checked and serviced as needed. Brake fluid should be flushed and changed every couple of years to prevent brake fade on long downgrades. Front flex brake lines may need replacement, can rupture inside causing sudden pull to right or left . Safe vehicle and safe driving are most important.
Good luck with new adventure/hobby.