As Donn just mentioned, the weight your using is the empty weight, I believe from last years model. The current 3120RL shows a shipping weight of 12,050 and has a carry capacity of an additional 3950, for a total weight of 16,000.
Even if you keep the weight down to 15k or so, the 20% mark is going to have you hovering around 3000 lbs.
Even at 14,000 your still looking at 2800, so plan your loading accordingly. Many on the roads are a little over, since your looking at a 10 year old truck, no matter how well its been maintained, it would be a good time to recheck your brakes, and consider a tranny cooler.
Happy motorin.
Edit add, that shipping weight doesnt include any water, it doesnt even include propane or tanks, nor does it include batteries.
