Better Gas Milage and Shift Lock on e-350 based RV?

Be happy with 9 MPG. Trying to do anything else will be costly with frustrating results.

One of the best alternatives is to always drive at 55MPH. You'll see another 1 MPG at least.

The worst thing you can do to an engine is to allow it to lug up a hill in top gear (waste of fuel and puts a strain on the drivetrain). The downshift will get you into a good torque range for going up a hill so you run more efficient. These aren't hot rods, even at only 21 feet that's a lot of weight you're carrying.