The miles per gallon that an RV gets depends on the size of the engine and the size of the RV.
The average large RV gets around 8 to 10 miles per gallon which is pretty good fuel mileage for an RV and all that weight you're driving around.
Some newer smaller RV's may get as much as 20 miles per gallon and if you do get up to 20 miles per gallon that's the best gas mileage that an RV can get.
Also if you're towing a travel trailer the larger the travel trailer the more fuel your vehicle is gonna use and you can expect between 8 to 10 miles per gallon when towing a travel trailer.
However if you're going traveling in an RV the cost of the fuel is worth it even though your fuel mileage is not that great.