Assume a brand new diesel one ton dually with exhaust brake. Truck has a 13,200 GVWR and a 9300lb rear axle. Assume a new tandem dual gooseneck with 12k axles and good brakes with a 7k empty weight. Trailer rated for 25,900 GVWR You could put up to probably a 20k load on the trailer (27k gross with trailer) and have a 4K pin weight on the truck and 23k on the trailer and be completely legal from an axle weight and tire rating standpoint on both the truck an trailer. Heck dodge rates their new one ton duallys to tow 30k goosenecks. I personally would not want to go that heavy but how close would you get or have you towed?
I would have no issue with it as long as the gooseneck neck is rated for it.
I had an Eager Beaver with three 7K axles behind a 97 dodge diesel dually. On several occasions had 20K plus on the deck, around 5K on the pin. Overload springs of course, didn't seem to mind at all. This truck did have adapters and 19.5 motor home tires on it which helped tremendously. The pin was far enough ahead of the rear axle that it spread the load just about perfect, not a hint of nose lift on normal roads.