I have an 01 F350 with a v10 that I tow my 3400 lb deckover hauling my offroad toys. It is lifted with 38s with 4.30 gearing so I get a little less than a stock truck's mileage. I average 10 mpg at 70 mph with the truck alone and towing I get 7-8 mpg at 70. I've hauled a Hummer H1 with little problem - the v10 does like to run in the upper rpms on the steep hills though when pulling heavy loads.
Here is my tow rig - as it was in 2001
As it was in 2004 (when the towed toy got smaller and the tow vehicle grew larger) :)