That sounds about right for the newer trucks - but are you sure about the tank size? I used to drive an automatic 02 Sierra as a work truck with a 105L~ tank and we could roughly get a 1000km if we behaved.
You're probably closer than I am. I should run out and check. I don't think it's as large as 105L, but closer to 80. Its only a half ton.
For what it is, and the HP and torque it gets, its gets pretty good gas mileage. That's the main reason we bought it over any other brand. Its actually the best full size truck with a V8 on the road right now. Its the same exact mileage as the 4.3 L V6 found in the old S10.