Payload includes looking at both axles and the GVWR.
5th wheel/gooseneck weight sits directly over the rear axle, so all the weight goes onto the rear axle. It can't take advantage of any payload boost the front axle provides.
Reality is a little more complicated but that's the main jist of it.
Be careful if you have replaced the tires or rims to make sure they don't have a lower rating than the stock items.
Also, keep in mind, since all the passenger and storage space is behind the front axle, any people or gear you put in the truck will at least partly wind up on the rear axle, so if you have a 3100lb hitch weight and 2 - 200lb people get in the truck, you will be over the weight rating on the rear axle.
If you are slightly over the ratings, the truck isn't going to go up in a ball of flames the second you put it in drive but you are pushing it hard and will have less margin for error.