We have a 24' C that is larger (taller/wider), than most, see sig. It sits on a 2011 E-350 chassis, V-10 with the 5 speed Torque Shift trans with the 4:10 rear end. No way we will ever see the mileage of a more aerodynamic B+ but then there is no way they have the interior room and storage that we do.
We cruise at 55 to 62, do not have a toad and I tread lightly on the go pedal which seriously improves our mileage as verified at the pump on every tank and constantly by the Scan Gauge. After 20,000+ miles in the last year (at the end of every long trip), we consistently average 9.5. This includes a lot of dirt roads, city traffic, shopping, sightseeing and yes, some generator run time. To average 9.5 you have to spend a lot of time getting 10 to 11+ and we do.
If you don't have a Scan Gauge I highly recommend you add one. The fuel savings that can be realized by merely paying attention will quickly pay for it. Just monitor average and real time mpg and try to keep the real time higher than the average, kind of fun to do actually. :B
The ability to monitor trans fluid and coolant temps is priceless and will allow you to make the most efficient use of your Tow Haul mode adding years to the life of your tranny. Those are just two of dozens of readouts it can provide and don't forget that it is a built in code reader as well.
:C