I pulled a 22FB with a 2010 Tundra, 4.6L, slightly smaller engine, slightly smaller trailer and averaged around 10mpg towing. My truck had the tow package. if your's doesn't and you have a higher gear, you may not be far off from normal. I also pulled around 60MPH, much more and mileage went to hell in a handbasket.
Now I'm pulling 8000lbs with a 5.7L w.tow package and getting about the same MPG. Not apples to apples, but I hope this helps.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner