I get the same mpg or better at highway speed. Its around town that it suffers. Since i drive 75-80, one theory I have is that the increase in tire size lowered rpm at that MPH, and therefore helps.Quite Possible depending on your gearing. Check my signature for my fuelly AVG. 15.8 MPG in a lifted 4" tuned truck on 33.6" tires. Can still get 18-20 MPG in the summer driving highway. So yes plenty of stock 4x4 trucks getting 20 MPG.
If you haven't had the computer calibrated for the increase in tire size either than you're truck is showing worse MPG than it's really getting.
A tune will solve that, fix the tire size issue and give you back a few ponies and maybe an MPG or two to boot.