I just got back from a trip to Steamboat Springs and had to climb two passes with my 2010 F 150 (5.4 V8) rated at 7400 Lb towing capacity, and 5.0 TA which is 4140 Lbs with the fresh tank full. The truck worked hard going up to the Eisenhower Tunnel; whenever I had to slow down behind a semi it needed to wind up to 4500 rpm to (very slowly) recover the speed lost. Probably just staying behind the big trucks going 45 with their flashers on would have been better on the truck, and gas mileage.
My intent here is to state that there is a world of difference between towing up and down mountain passes, towing at higher altitude and doing it with a strong headwind or crosswind.
Ford is supposed to come out with a 2020 Hybrid F 150. If that is paired with the 3.5 Ecoboost engine and 10 speed transmission I will stand in line to get one.