I'm in the 12-13 range, mostly city with some highway on my 07 5.7L, added mods/weight and agressive tires.

But im not heavy on the throttle, though i don't granny it either.

Depending on the mileage, you may be due for plugs and other maintenance that'll revive some of the lost mileage. If that 5.7L is stock, i bet it needs some plugs, pcv valve and some fluids changed.