What gear are you running? Try putting your stock tune back in. Not all tuners are good for fuel mileage.

BTW, your truck is working against you when it comes to fuel mileage. It's heavy, tires are heavy, toolbox is heavy, transmission his heavy and geared not so great, wide tires create drag. You're probably doing good for a heavy pig truck. Max out the tire pressure to the max posted on the tire sidewalls. Lose the weight in the bed of the truck unless it needs to be in there all the time. The rig needs to go on a diet if fuel mileage is a concern or you need to trade it in on a diesel.