I disagree that K&N doesn't do anything for gasoline engines, but you need to decide what all you are going to do before doing anything. Reset the computer if you haven't. If it was idling a lot, that counts in figuring mileage.
Go search at Ford-trucks.com. There are a lot of reasons why the mileage could be so bad including some type of engine problem, clogged fuel filter, old gas, dirty carb, dirty air filter, brakes dragging, low tire pressure, clogged or damaged catalytic converter, etc.
They can also tell you how to read the VIN to determine what gearing it came with stock.
If it was mine, I'd do the following. Air up the tires, change the exhaust to a larger diameter pipe including the catalytic converter, change the air filter to a more free flow type, add a tuner that takes advantage of the air filter, and has a better shift logic. After you run out the gasoline in it, or if it's at least low, run some Seafoam through it.