Same here with us we get 10.5 to 11.5 mpg towing and with a good tail wind add 1 mpg. I realy cant say that is my best yet because it is only a baby with 7 k on it. I know it will get better as it ages as my Ford did the same. the most I received not towing and driving 62 mph I got 17.6 mpg. I have a 3500 quad cab and have 410 gears and DRW, which drags the milage down compaired to the single rear wheel.
Here is some reading material that was sent to me the other day you might enjoy the read.
http://list.priceweber.com/cummins/td/mpg_shootout.html