I have a 2002 Tahoe with the 5.2 litre engine. Pulling my 3500 pound camper I get about 12 - 13mpg on the interstate.
Around town if I take it easy I can get 14 on a good day.
Big vehicles just burn more gas.
You can pull the engine and put in a diesel. Friend of mine did this with his Suburban. Got better mileage
but wasn't sure he would recoup the expense.
All the Best,