Mine's an '04 with a 6.0 in it, but it's the 2500HD. The difference between the Heavy Duty package and just the 2500 is the rear end ratio. If there's no HD after the 2500, then you've got the rear end with the 3:18 ratio, which gets better gas mileage than mine, with the 4:10 ratio.
I get roughly 14. That's in town and on the highway. Makes no difference. With a trailer, I'm lucky to get 8 mpg.
btw, mine's a 4-wheel drive too. That'll make some difference as well.