You sure don't want to go to smaller dia tires....however, I would think that the tires on the mh are stock tires that the mfg put on there and would be matched to the weight, rear end ratio, and engine torque specs. Going to a smaller dia tire will lessen the mpg, not increase it due to the engine rpms being higher for the same speed.
I don't think I would vary from the stock size/load range of tire.
The previous owner could have had a heavy foot and didn't care about mileage; hence, the low overall mileage. He also could have been pulling a rather heavy trailer/toad.
I myself, will drive fast enough to get the trans to shift into 6th and then back off two or three mph still keeping the trans in 6th. This seems to work best for me and my cat has an overall average mpg of about 8.5.
Ron & Sandie
2013 Tiffin Phaeton 42LH Cummins ISL 400hp
Toad: 2011 GMC Terrain SLT2
Tow Bar: Sterling AT
Toad Brakes: Unified by U.S. Gear
TPMS: Pressure Pro
Member of: GS, FMCA, Allegro
RETIRED!! How sweet it is....