We have a 2010 2500 Duramas with the Allison transmission towing a fifth wheel that weighs 11,280 loaded and ready to go. We get 10.5 to 11.5 towing at 62 to 63 MPH. I would guess that MPG not towing would be in the 17-18 MPG range but we only use it for towing so am not sure.
We just returned from a 4,500 mile trip through TN, GA, SC, NC, KY, OH and IN and for the trip averaged 11.2 MPG. That includes towing and what ever local miles we drove. The worst actual towing days was 9.8 into a very strong head wind and the best was 12.1 under almost ideal conditions in IN.