Try like 9-10 mpg when towing with the 5.4........ And I only have a #4500 travel trailer! I do get around 14-15 mpg when not towing.
The 5.4 is a great engine, but consider the fact that an F250 weighs probably a good #1000 more then an F150 even before you load it up.
For a #6000 trailer, that would be about the most I'd tow with the 5.4 no matter what 'frame' it was bolted to.
I get pretty decent performance while towing my trailer, but you know it's there and forget about running in OD.
I towed with an '89 E250 van with the 5.8 in it. Try around 6 mpg. It only got 12 mpg when empty. I think that's why they gave 'em two tanks! I remember only getting around 150 miles per tank. This was towing a #6000 utility trailer from coast to coast. It did ok on the flats at 60 mph, but it really struggled on the hills. Sometimes I was in first with my foot to the floor and moving at 25 mph!