Half-ton Chevy owner here, with the 5.3 V8. I think you'll find that with any gas motor, towing any trailer (a wind-grabbing parachute) your average gas mileage will be anywhere from 8-11mpg, depending on terrain and wind.
I towed a large Toy Hauler with my half-ton, almost overloading it. I averaged 11mpg. I sold it and got a small single-axle travel trailer, weighing about 2700 pounds. I still averaged 11mpg. I settled on a mid-size 24' travel trailer weighing around 6000 pounds, and I still averaged 11mpg. (sometimes more or less with each trailer, depending on terrain and wind) That just confirmed to me that the truck didn't really care about weight when it comes to MPG; its all about wind-resistance. Each trailer had roughly the same square-front, even though they were varying lengths and weights I got the same MPG. A few thousand pounds didn't mean anything to the truck except for faster take-off or stopping. Once at cruising highway speed, its all about wind resistance.