Well as the OP has probably figured out, when in doubt go bigger.
That is okay if you willing to pay more for initial cost, repairs and maintenance; plus you are willing to except a rougher ride when unloaded.
Every feature you add to any truck reduces the max cargo, be it power windows or heated seats. That is why the dealer can only give you a rough estimate of the payload.
There is nothing mystical here, a half ton is 1000 lbs. The Max cargo on my truck is 1411 lbs. that means after a couple of adults and a dog I am eating into my cargo capacity. If my TT and stuff exceed the max cargo, I either tow over capacity or buy a bigger truck.