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

Aerodynamic drag builds up exponentially as speed increases. Trying to keep up with cars and big rigs usually means trying to go over 65. Relax and drive a steady 55 to 60 in the right lane when you can.(The right lane is often a rougher ride than the left lane on worn, cupped concrete highways though)
9mpg on relatively flat terrain is good, expect 7mpg in somewhat hilly country.