For the setup in my signature:
Minnesota to Maine, and back averaged 11.3 mpg
Minnesota to California, up through Oregon, and back - 10.5 mpg
Minnesota to Mobile, AL, then Louisiana and back - 10.3 mpg
Usually drive 60-65 mph.
In each, there was some "off hitch" time, but not a significant proportion that would swing the numbers much.
Worst was going to CA - through South Dakota ran 5.5(!!!). Stupid 20-25 mph headwind! But made it up in other portions of the trip.
BTW - Ford recommends premium gas in the Eco for towing. You get about 10% better mileage. I always do that when towing. I use regular for general use, except when premium is less than 10% more in price, then I switch it up.
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder