If you want to get a little better mileage, cruise just above the shift point to overdrive. You need to have enough RPMs to avoid lugging, yet not enough to use too much gas. About 300 to 500 RPMs over the shift point should be fine. If you find the RV hunting for gears, you need to increase your speed. For me (Honda Accord), I get my best mileage at 55 MPH (~1800 RPM). Your RV is likely different, though.