Ok, can't resist. I'm at home and bored with everyone asleep.
GMC 4500 chassis GCWR 20,000 lbs (2008 MY). Your dry weight (no fuel oil or fluids) is 9500. Taking as it is written, your trailer weight is 7000 lbs empty so the combo is 16,500 without any fuel, fluids, or freight. Leaving 3500...