The miles per gallon that an RV gets will depend on the size of the RV since larger RV's will use more fuel per mile than a smaller RV.
Smaller RV's may get around 10 to 16 MPG while larger RV's may only get around 5 to 8 MPG.
An RV will certainly use more gasoline or diesel fuel than your average vehicle so you'll need to budget some extra money to fuel your RV trip.
Some RV's might get 20 miles per gallon which is pretty good but some get less than that.
So be sure to fuel up the RV at every opportunity to avoid running out of fuel on the highway.