I have a 2016 2500HD crew cab, 6.0L, 4.10, 6 spd and tow a TT with a GVWR of 11,070 (about 7100 dry). Avg MPG towing is 8-8.5 and local truck only driving is 13.5. Truck alone would be better on a trip probably around 15-16 MPG but haven't taken any empty trips. I bought the truck to tow and for local grocery getter / parts trips. I have 3 trailers (2 14K and the TT). Anyone who tows would love to get better MPG. I don't worry about it. All of my above list MPG has been local back road driving, no highway miles. Its possible more highway miles would raise the overall MPG.
On a side note I previously had a 2500HD Duramax that would get 19MPG empty and 14 MPG towing 7500 lbs on the highway. It was a lease truck which I should have bought at the end but for some reason I didn't and I couldn't justify the added price buying a diesel in 2016 for the short runs.