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 available (with no driver, fuel, fluids, or freight) before GCWR, which doesn't matter because the maximum weight the vehicle was designed for is not a legal thing (like that over 14,000 GVW).
You don't forsee any extra maintenance, wear and tear, fuel consumption, or handling problems putting 10,000 lbs on top of that 16,500 (plus fuel, oil, fluids, straps, you, and anything else you throw in there? Oh, it doesn't matter as long as you don't overload each individual unit. And the brochure shows "--" for GCWR meaning unlimited, perhaps?
Dual rear wheels = DOT attention, no trailer necessary, so to your original question...dude are you insane