Some things you can do to improve the gas mileage in your truck are to clean your fuel injectors, change your spark plugs, spark plug wires, ignition coils, keep your tires inflated to the proper level.
Also avoid pressing too far down on the gas pedal to go faster as you use more gasoline the faster you go and don't use high profile tires or too large of tires for your truck as they may look nice but your gas mileage will suffer due to the too big of tires.
Keep your air filter clean and replace it often as well as a clogged air filter makes the engine starve for air which can cause the engine to not burn the fuel correctly which reduces your gas mileage.