Weird....my gas mileage has been pretty low on this truck since I got it. I'd attributed it to the AT vs my old trucks MT. (and the fact that I've got 6" of lift, 33's and a whole bunch of extra weight) That said, I've read your other thread regarding low idle, and mine exhibits that trait at times as well. I wonder if the two are related?
Out of curiosity, when you start your truck, are your lights dim until you hit about 2700RPMs? I've had that problem on my truck since day one as well, new battery, new alternator, and checked all my connections....yet, it won't charge until I've hit 2700RPMs once, then I can idle all day and it'll maintain a charge.
If I find my solution, I'll report back.