I tow a 6,000# (Wet) Nomad with a relatively small Ford 4.6L Three Valve in my Sport Trac.
Best MPG with trailer is 9, mostly on flats @70-75MPH, worst is 6.9, and that was climbing four 6% grades and trying to keep it over 55 on those climbs...
Around town, no trailer, with regular gas, 11-12MPG, and with Premium and a different performance tune (I have an SCT Programmer) I get 13-14. The Premium pays for itself as the cost is about a wash, but the truck is much more fun to drive.
I am a lead foot, It's a by product of driving in Europe a lot, and I do 80-90 on the freeways on the way to work.