Well, this thread has devolved into the usual "how big a truck" debate, but to report to the OP my own specifics:
2012 Crew Eco, Max tow, Max pay (3.73 gears). Just returned from a 250 mile each way trip requiring towing the 8,300 lb fifth, return empty, then tow the 3,500 lb boat. All highway - combo of speed limits of 70 mph, 60-65, and small town 35 mph stretches. Yes, did the route 6 times in two weeks!
Non-towing: 20.5 mpg one way, 21.8 the other.
Pulling boat: 17.2 one way, 16.8 the other.
Pulling Fifth: 8.8 one way, 9.1 the other.
As daily driver, non-towing, around town, summer average 16.5, winter about 14.5
With fifth wheel:
Trip Minn. to Maine and back, 11.5 overall
Minn. to Calif, Oregon and back, 10.3 overall.
Minn. to Mobile, AL and back - 10.1 overall.
When towing the fifth, wind makes the biggest difference by far. Was bucking 20 mph winds both times on the latest trips reported above (why are they headwinds going both ways???)
Ford notes, and my experimenting agrees, that you can see up to a 10% improvement in mileage by using premium, which I usually do when towing, except for this last round of trips, as the price difference between regular and premium was far more than 10%, and I didn't really care about getting the additional range before refueling.
On the Mobile trip, a cracked connector disabled the turbos so yes, suddenly it became very hard to tow the fifth with what became a small V6! But otherwise, all has been fine. Have done many 6%+ grades with the fifth up and down for long distances with no issues. Can pass anyone I want, going up, if I want. Engine braking going down in tow mode worked well (but I cannot compare to a bigger engine, so can't say if it is better or worse).
