With a 2001 Duramax, when a drop in mileage happens, the first thing you have to do is check the injectors. The 2001 through beginning model year 04 had injector issues, so bad that GM increased the warranty on injectors to 7 yrs 200,000 miles. Smell your oil and see if it smells bad like diesel fuel, and or seems to be fuller than it should.
With that said, the new fuel blend is not the best, do to the ULSD. I have an 04.5 (LLY Duramax engine) and have been running Stanadyne Performance additive since it was new. Stanadyne will help with the lubricity issue as well as other performance increases.
I just cleaned my injectors with Seafoam. Between cleaning the injectors and a good aircleaner I picked up 2 mpg. I have checked mileage at each and every fueling since the truck was new by filling up to the neck and hand calculating mpg. 51,000 miles worth of checking so I know every change that affects mileage.
Here is a good site about Duramax diesels http://www.dieselplace.com/forum/
There almost always is a reason for mileage drop. This summer we did a trip from Akron Ohio area to the Outer Banks. 1400 miles round trip. We lost 2 mpg towing (before I cleaned injectors and air filter). I had put a new air filter in. Well it turns out that new air filter had a significant hand in the loss of mpg.
I also run a pre-filter in between the primary filter and fuel tank. I can tell when the filters get dirty by a loss in mileage. And it has happened relatively soon after a new set was put in. I just happened to get a load of dirty fuel.