Yours will be too new.

Couple of reports I've gotten recently:
2004 2500HD extended cab, 6.0, 4wd - 75-80mph towing flatbed with 5000lb Econoline van on it = 10-11mpg
Also 28 ft enclosed same fuel economy, but economy drops to 9-10 if the speed drops to 70-75 (yes, I double checked, evidently above 75 is a sweet spot that keeps the tranny humming along in overdrive and doesn't require much downshifting) Only mod is K&N intake but that had a noticeable fuel economy benefit (almost 1mpg).

Met someone at Barber this weekend who tows a 2 car open trailer with a 2000-2003 generation 2500HD extended cab and said he liked it better than the v10 he had previously but did not track fuel economy.

BTW, I've heard good things over on the gm-truck forum about some ECU tuning that helps with fuel economy.
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The one that I am getting is the Classic, so it shouldn't be that different? I too have seen some info on some Cals that seem to help these trucks out.