Many factors will determine gas mileage. Doing a search will net a wealth of info but here are the things I would do first.
1) Maintenance- Pull the plugs and see if they are clean and gapped properly. Use Autolite or Motorcraft copper cores if replacement is needed. While the plugs are out a compression test will tell you the basic soundness of your engine. Change out the wires, rotor, and cap if there is any doubt about their condition.Change the oil to 10w-30 with a Motorcraft filter. Change the fuel and air filters if they are due.
2) Check for dragging brakes or misaligned front end. Aggresive large tread tires will also hurt Mpg. Make sure tires are aired up to spec.
3) If your service truck was like mine you might be carrying 500 lbs or more of extra weight. My old 92 F150 4x2 would get 13-15 Mpg.
4) Aerodynamics -Pipe racks or anything hanging out in air stream will hurt gas mileage too. Check to make sure the originial airdam is still on the truck.
5) Driver habits are the biggest factor in gas mileage. Excessive idling and speed will naturally hurt Mpg as well as hard acceleration.