Reading your responses since my first answer, it bothers me that you get only 15 mpg driving a F-150 with the 4.6 V-8. I do better than that with my bigger, heaver E-350 van with the 5.4 geared for a heavier gross and tow weights.
Your 15 MPG is saying things about your driving that do not bode well for good MPG towing. If you want to see much better than 8-9 MPG towing a "regular" travel trailer with 80 sq ft frontal area you might have to slow down, accelerate more gently.
12-15 MPG is achievable towing a smaller or collapsible trailer, 50-65 sq ft, but that is for vehicles and drivers accustomed to getting 18-24 mpg from those same vehicles when not towing.
