.... The tank has to be filled every 500 to 600 miles with DEF(diesel exhaust fluid) and if you let it get to low it puts your truck into LIMP mode until you fill it again. So this cuts into your fuel mileage a bit, but no one talks about that. About 25.00 dollars to fill it.
Not to be terse, but your info is skewed.
Speaking only for the GMC / Chevy - In rough terms, the 5 gallon DEF tank is supposed to be good for about 5000 miles. There is no guage or dipstick to know how much is in there. With approximately 1000 miles remaining till empty , the truck (DIC) prompts you to refill the tank soon, which should cost less than $45. At 200 or 300 miles remaining, your speed is limited to 55 mph and the nagging gets more persistent. When the tank runs completely empty, your vehicle will be limited to only 4 mph. There is a sensor in the tank that monitors the "quality" of the DEF so that is can't be diluted or filled with anything other than the "real Stuff."
Your fuel mileage is totally unaffected by the DEF system. The DEF only reacts with the Nitrous oxides in the exhaust to break them down into ammonia.
The 2011 Duramax gets about 15% better mileage than its predecessors, as during the "Regen" cycle there is another fuel injector that injects diesel fuel into the exhaust to clean the particulate filter, whereas in older trucks the engines fuel/air mixture was richened to raise the EGT to clean the particulate filter. The regen now occurs about every 700 miles as needed to clean the exhaust filter, rather than every 400 to 500 miles as previous models did. The regen cycle is less frequent, shorter, and doesn't have to run the engine in a richer condition, so performance is not affected and there is no risk of engine oil dilution, as in the earlier models.