This is what GM puts on their website for the 6.0L "Standard in 1500HD, 2500HD, and 3500 models, which are not rated for fuel economy." An extended cab 5.3L is rated at 15city 18 highway not loaded. Mine gets a little under 15 city and varies from 17 to 18.5 highway (5.3L 1500 Z71). The only way to get better mileage out of a 6.0L is to buy a different truck. Those numbers sound correct for what you should be getting. You can throw lots of money at it and get maybe a mile or so better with chips and what not. I think the duramax/allison trucks get much better. Sorry, but as you said you have a 6.0L gas pot
Edit: You may be able to find a lot more information here: gm-trucks.com