I had an 06 crew cab f150 with a 5.4 that avg 14.7 mpg driving mostly in town and some highway. Didn't do much towing with this truck besides a few cars on a car trailer since I also have an older dodge diesel I tow with. My dad wanted me to borrow his 09 crew cab f150 last summer for our road trip from western Washington to South Dakota since they were thinking of buying a trailer to tow around with and he wanted my opinion. I will say that I usually tow with my 96 dodge diesel and I average 12-13mpg towing my 26 foot toy hauler. So my opinion on an 09 f150 crew cab with a 5.4 goes like this:
Unloaded- The truck drives great, nice a comfortable ride, decent fuel economy (about 14.5 in town and 16.5-17 on highway,) all around great truck. His truck is absolutely fully loaded so it was great compared to my dodge.
Towing- If all you had to do was tow something on flat ground and more aerodynamic then my box of a tow hauler it may do OK. By the time I got from Tacoma to eastern Washington I was ready to pull into Dave Smith on my way through and buy a diesel and leave his truck there. My wife convinced me not to though and by the time we got home 2 weeks later I think she regretted it. I averaged between 5.5 up to 6.5 mpg if I was lucky. On flat ground it towed good, had the power to pull and was stable. Once you got into any incline forget it. Smaller hills were manageable if I could time it to not lose too much speed before I got close to the top. Mountain passes flat sucked. I tried every combination of gears and towing options his truck had but nothing was good for passes or long inclines. Once it started losing momentum it slowed down so fast. Once the truck slowed down enough it would then kick down and rev to all hell and you would think the motor would blow if you kept it there so then you take your foot off and you slow down even more. Very, very slow was the only way over a mountain without thinking you would destroy the engine on the truck and I saw mpg numbers on the truck computer dip below 3.5 mpg on a lot of the trip, the flats and declines were the only saving grace to get what I even got on average.
I know my trailer isn't what you have in mind for towing so a smaller horse trailer or even a smaller Airstream type will do better due to aerodynamics. This was just my experience with this type of a towing situation and I was not impressed at all. If I was to buy an f150 for towing a lot of heavy loads I would probably be buying a supercharger, intake, exhaust, programmer and regearing the truck.