I own an f350 - i know you DONT get 15mpg (around town) or more than 17 on the highway - EMPTY.
Ive got several friends that didnt listen - bought f150s and are pissing and moaning about their 6-8 mpg when towing LIGHT travel trailers. A friend of mine just bought a new f350 and he's pleased w/ his 11mpg (over the 6mpg he was getting in his suburban) !
towing our toad costs us a 1/4 of a mpg at most - sometimes we don't even drive it.