I could say we get 13 mpg on the highway, but I prefer to provide "TRIP AVERAGE" figures with consideration to origin, destination, and other factors.
- we live near Chicago so we travel far to nearly every destination, most often driving across the Great Plains
- our practiced cruising speed is ~65mph
- we run our generator as needed to charge our batteries and such which reduces the calculated figure
- we most often tow a vehicle and use that one once at our destinations, making our RV driving mostly on the highway
So....
Our 2007 Phoenix Cruiser 24' long B+ (2007 E350-V10) with tow vehicle, our TRIP AVERAGE is....
~10.5 not towing
~9.5 towing

Our 2007 Ford E350 with the 6.8L-V10 engine has the 5-speed TorqShift transmission which was installed in all E350/E450s from 2006 to 2016. Starting this year 2017, Ford installs a new 6-speed transmission in their DRW cut-away E350/E450. It offers better performance & behavior, and "Maybe" a ~10% improvement in fuel economy. If it is all true, I'd love that transmission.