2012 E-350 24' Class C with the V-10 5 speed Torque shift trans: After 32,000+ miles from coast to coast and then some over the last three years:
Cruising 60 = 9.5
Cruising 65 = 9.25
Both of those numbers reflect a little generator time and much of the mileage was in the mountains of the western half of the country. If you are not running with the Tow Haul engaged you should be.... that is what it is there for and it will not hurt your mileage.
I should also note that those numbers do not change when I am towing my motorcycle, bike and trailer are less than 1,000#. The E-450 chassis is 3,000# heavier than mine and has a lower rear end. IIRC the 450 is a 4:56 vs the 4:10 on my 350.
:C