I used to be pretty active on Ford Truck Forums and have owned a V10 Superduty for over eight years now. I've heard and experienced the opposite. The mileage changes very little towing or not. With the available gearing (at least when I bought my truck) it actually has more horsepower and torque to the ground, with little to no drop in mileage. The V8 (gas) though did have a significant drop in mileage towing in the same truck.