First off, I think you'll find this Edmunds Topic on the DURAMAX very interesting, and you should join in!
Secondly, yes you probably will lose fuel mileage with larger (and wider) tires.
Did you have the dealer recalibrate your computer to compensate for the larger tires? If you didn't then you are probably getting better mileage than your calculations would indicate, because your odometer is off.
From what I've read, with a STOCK Duramax, you should be getting 10-12 while towing and 15-17 on the highway in high gear with an empty truck.