All smaller gas V8' will eat gas when towing. If you have to travel great distances, maybe the diesel is the best choice for you. How many miles a week are you towing?
My 95 F150 5.0L five speed will also knock down about 15 mpg without the trailer and 8-9 towing (1200lb trailer, 2200lbs of mowers. A diesel would do better but I don't need to tow that many miles.
Also, what is the rear gear ratio in the truck?