3.08

I have a 2011 Sierra 4X4 SLE with the Off Road suspension pkg and Power Tech Package which includes the 5.3L Vortec V8 with heavy duty cooling but has a 3.08 rear end. My problem is towing a 3000# Wells Cargo with 750# of cargo. No trailer and I get 15mpg. With trailer and it's 7mpg. What...