I am just about to place an order for a 2021 Sierra 3500 AT4. I plan to pull a 5th wheel (not yet purchased). I want to provide myself as much payload as possible in this model so as not to limit my 5th wheel choices any more than necessary. My question is in regards to long vs short bed. Intuitively I would think the short bed would have a better payload (weight not volume) as it has the same suspension and a smaller (less material/less weight) bed, but the specs say different. The specs state that the payload of the long bed is a bit better. Can anyone explain to me from a physics standpoint why this is the case? I'd love to be told this is not true (which would convince me that the easier to park in a parking lot short bed would be a better choice) but, given the 5th wheel is my main reason for buying the truck, I will stick to the long bed if the specs are correct. Thanks in advance for the input.