I have an 04 6.0 also but the F350 FX4. So basically same truck, need to know if yours is 2wd or 4x?. My TT is smaller but I get around 11-12 with it so you are pretty much in ball park I would think. I run a hypertec economiser tune on the B setting and it seems to help. I also have a turbo back gibson free flow exhaust system. These are the only mods I run. Greater concern for you should be are are you monitoring your engines vitals? EOT, ECT, FICM voltage, 12 Voltage? Others are EGT, boost and fuel pressure. PM me if you have any specific questions to this.
I have a friend who had a 03 dodge and he pulled a 5er with it and he only got around 9 maybe 10mpg. He only had a free flow exhaust I think his was close to your weight also. But remember the ford is a heaver truck to begin with. My 4 door is around 7700 lbs by itself. Slow accell is supposed to help as is going slower is what most will tell ya. Lessen the weight in the trailer if ya can (momma wont be happy I am sure), run tires to max pressures (truck and trailer) all the normal stuff.
My truck gets as good a fuel mileage not towing at 80mph as some trying to stay around 60-70mph. Wife hates it when we dawdle going anywhere. Made me try 70 mph in our new to us MH, .5 gal difference overall!
