Welcome, T_B. It's tough to get decent mileage from a 2-1/2 ton FSP. You can certainly top 20 mpg on long highway trips by keeping your speed at or below 60 mph, but it's really tough to do it in city driving. Just getting your F-150 rolling means you're getting <5 mpg from every start. The way to do it is ease off the gas after it's rolling, and coast everywhere without touching gas or brakes. It's tough to do.
Try adding about 10% Diesel. I say about 10% because you will need to play with it a bit to get the best MPG. For what ever reason this process varies from vehicle to vehicle. This increases toy MPG because diesel typically releases about 38.6 MJ/l (138,700 BTU/US gal), whereas gasoline releases 34.9 MJ/l (125,000 BTU/US gal), 10% less (http://en.wikipedia.org/wiki/Diesel_fuel) or diesel burns hotter than gas. Adding the 10% will not effect or damage your motor either like some potions and elixirs being sold will. I went from an average of 14 MPG to 18 MPG just by adding the diesel for the past 30K with no problem.