Bob, I've towed my race car on an open trailer with a 2005 F150 4X4 with the 5.4 liter engine. No problems, got about 9-10 MPG when towing. Probably about a 6000 pound car and trailer.
With my new F150 FX4 (new as in 2014), 3.5 Ecoboost engine, I tow my fully enclosed trailer with the car and a bunch of other stuff. I am right up at the max recommended weight, just over 10,000 pounds for the fully loaded trailer. That engine is unbelievable as far as power, it has absolutely no problem towing that trailer. However, mileage drops precipitously, from around 17-18 for just the truck to 6-7 when towing the trailer. Also, my trailer is 28' and it acts like a big sail in the wind, it jerks that half ton pickup around pretty good when its gusty or when some big semi goes by at speed. It gets me there, though...