Keep trying! Apparently it can be done....I got 22.5 MPG, all stock, on highway on my way to Moab the first time. I re-checked my numbers and it seemed to be accurate....
Of course now I've added like a zillion lbs to the truck in mods, (steel skids, bigger tires) and am about to add 90 more lbs in steel rock sliders. Best I get now on highway is about 17.5.
Check out this site: fuelly.com
You can keep track of your fill ups/mileage and price.
Here's my profile: http://www.fuelly.com/driver/vxkat/vehicross