I know you are asking about the Duramax, but for comparison purposes, our 2005 Ford/Quigley 4x4 E350 Super Duty with the 6.0L diesel is rated for towing 10,000 pounds, and gets around 15 mpg around town, close to 20 on a long trip on the Interstates, but just under 10 pulling our 8500 pound trailer. It does have the 4x4, 4.10 gears, and oversize tires which all reduce mileage ....
Our old 1995 Chevy/Quigley 4x4 G20 with the 5.7L gasser and 3.73 gears is rated for 6700 lb towing capacity. It does about the same as the Ford when it is not towing - 15 in town, 20 on the highway - but we were lucky to get 8 mpg with a 6000 pound trailer. It also worked hard to get over about 55 mph, and often shifted to lower gears on grades or with headwinds. On one trip to the Outer Banks into a 35 mph headwind, we could barely maintain 40 mph, and only got 5 mph. The Chevy has been retired to surf fishing duty.