JSYK, the 5.3 motor will get better mileage then the 4.8, it doesnt have to work as hard, tire size or gearing will effect MPG.
if you hate the chevy/gmc for gas mileage, then your screwed, the fords and dodges get worse besides the eco boost, and you wont find on of them under 35 grand for a while.
Most gas mileage is controlled by your right foot, the easier you are on the gas the better the mileage. make sure your MAF sensor is clean, as well as your fuel filter (04+ dont have fuel filters. its in the tank.)
that being said, your driving a truck your never going to get great gas mileage.