As a comparison the (2) F-150s I had in the configuration you described, both with the 5.0 V8 averaged about 18-19 on the highway, 15 in town, 10 while towing my 26' travel trailer, and 11-12 or so towing my 3-horse trailer. Power was always adequate.
That 5.0 is the best engine ever put in an F150.
Without a doubt, one of the best engines ever made. My last 2 F-150's have had the 5.4, and I've had good luck with them. One of them would get 18-20 MPG highway, the other maybe 15-16.
