Has your mileage improved? If not, take the rig into the dealer and have them run diagnostics on the engine and tranny. Our 4x4 Limited is getting close to 30mpg all the time. Towing a 5'x8' trailer with 800 pounds reduced mileage to 25mpg, but on relatively flat traveling and at 55mph. If you are only getting 18, well, something's wrong in Kansas, Toto.