Do you have larger than stock tires? Tires inflated properly?
Have you cleaned the MAF sensor? ---changed the air filter?
You have the DPF? If so it is possible that it is regenerating a lot, and that uses fuel.
Do you have a long bed-short bed- ? 26 gallon on the short, and 34 on the large.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.