It would probably help to post some specs on your truck (i.e. Engine size, Prerunner/4x4, RC/AC/DC, lift/tire size, etc....)
I drove from western Washington (State) to Clovis, CA before Xmas and got about 18mpg. On the way back I had close to 400lbs of firewood and other stuff and got between 15.5 and 16.5. Almost the whole way was on I-5 and I drive fast.