Non-towing - driving 60 - 20 mpg, driving 70 - 18 mpg
Towing - driving 55-60 - 11.5 to 12.0 mpg, driving 65-67 - 10.0 to 11.0 mpg.
Mileage towing will be impacted by head winds and hills/mountains.
My 2011 D/A gets about 1/2 mpg better than the 2005 D/A I had - 5 speed Vs 6 speed allison.
Previous DAs:
2005 Chevy Silverado 3500 LT CCLB LLY SRW 4X4
2004 GMC Sierra 2500HD SE ECLB LB7 4X4
The Duramax/Allison will tow that size of trailer without any problems but you'll probably be exceeding the GVWR of 2500HD truck (One of the reasons I traded my 2004 for a 2005 and the 2005 for a 2011)
If you can get a newer (2011 and newer) 3500HD SRW. GVWR is 11,600 lbs. The newer 2500HD has a GVWR of 10,000 lbs.
The "older 2500HD has a GVWR of 9200 lbs and the 3500HD has a GVWR of 9900 lbs.
2017 Keystone Avalanche 320RS
TV - 2011 Chevy Silverado 3500HD LTZ CCLB Duramax SRW 4X4