The manufacturer actually sets the GVWR of the trailer. It just cannot be more than the weakest link such as tires, axles, frame, etc....
The 3500lb. capacity axles originally used on the 21's were overkill. We did not feel comfortable rating the unit that high, thus we set the GVWR at 5,000lb.
The original 2" coupling was also rated at 5000lb, which may lead one to believe it was taken into consideration when setting that GVWR.
Sense then we replaced the 3500lb axles with 2500lb axles and standardized all couplers to a 2-5/16 ball the same used on our toy haulers.
That particular coupling is rated at 14,000 lbs. The 14,000 is the maximum tongue weight that coupler is rated for.
-Dan