From door stickers on 3/4 ton model trucks. A lot of fully optioned diesel 3/4 ton trucks that have a payload capacity of 1800# while a moderately optioned gas 3/4 ton can be rated at 4200# payload.Right, so still a limiting factor. Diesel models in general are rated to tow more than comparable gas models
Also, where are you getting that info? Diesels generate way more torque on average allowing for pulling heavier loads
A 3/4 ton is limited by the payload capacity of the truck the payload capacity dictates the pin weight.
Diesels are heavy everything on them has to be bigger and heavier to handle the torque.
You have to reduce the amount of payload available for a trailer pin, by the weight of all items in the truck . Passengers and hitch , tools and all items carried in the box and cab.
5th wheel and goose neck trailers, typically place 15>25% of their weight on the truck, usually closer to 20% a 10,000 lb. trailer can place 1,500>2,500 lbs. on the truck.
Payload of the truck is determined and limited by the springs, tires , strength of the frame, and other factors.
Very seldom is the limiting factor horsepower or torque in a modern 3/4 ton or single rear wheel 1 ton.
When you add dually tire 1 ton that changes the tables turn in favor of the diesel but you often end up over weight on the front axle before you can reach manufacturers payload.