The last time I looked, the truck plates have to be rated for the weight of the truck, passengers, cargo, trailer, and trailer load.
If you pull a 14,000 pound trailer with a 6,000 pound truck,, the truck plates have to be for 20,000 pounds,,,
The trailer plates are meaningless, typically they are not weight rated.
If I buy a Freightliner tractor for pleasure use, not hauling,, and it weighs 8,000, I do not need more than a 10,000 pound plate,,
If you hook a 40,000 pound trailer to a car,, the car better have plates for the gross weight, like 50,000 pounds.
The tow or haul rating of the car/pickup is different than the plates.