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

Those downshifts keep the torque converter cooler. It is better for the mechanical gears to multiply torque than pouring a lot of heat out of the converter doing it.

For overall financial perspective I'd say keep the speeds down, engine tuned, tires inflated properly, and buy gas at the best price. Beyond that sinking money and time into squeezing out the last tiny % probably won't give good returns. Your 9 MPG sounds in the ballpark as is.