Most large recreational vehicles average 10 mpg in gas. Class C RVs usually average a little better. Class B RVs have a slightly lower miles-per-gallon average than the van on which they are built.
When using a travel trailer, a traveller can expect a wide range of averages. A small travel trailer might impact fuel consumption slightly, but a larger travel trailer could bring the fuel consumption down to 10 mpg. Smaller Class C RVs average 10 to 15 mpg. Larger Class A motor homes average 6 to 13 mpg.
Diesel RVs get slightly better mileage. Diesel engines also have better towing capacity. Fuel economy chips help increase miles per gallon averages but only by 1 to 3 mpg.