I took a look underneath and made some measurements. The mounting bolts are about 4.5 inches apart and the receiver pin hole is about 9.5 inches from the rear mounting bolts. My normal drawbar puts the ball another 9 inches back.
With a 500 lb load, resolving moments around the rear mounting bolts, 18.5 x 500 = 4.5xF where F is the total upward load on the front mounting bolts. F= 500 x 18.8 / 4.5 = 2056 lb (an approximation, but not far off). Resolving vertical forces, the downwards force on the rear mounting bolts, R=500 + 2056 = 2556 pounds. These are the stated upper limits for attachment to the van and presumably includes a safety factor for bumpy roads etc. Note, with a load leveling hitch properly adjusted, there would only be a 250 pound downward load on both front and rear bolts (depending on the distance back to the trailer wheels). In all cases, loads would be half on each side for the left and right mountings.
I have a 14 inch receiver extension that I use occasionally with a light trailer so I can open the tailgate. With 500 lb on the ball, using the same calculations, F= 3611lb and B=4111. To keep the forces on the bolts within specifications, I would have to reduce the tongue weight to 56%. The warning on the extension says 50%.
I also have a hitch carrier which I have rarely used. It has a 7 inch drawbar and goes back another 20 inches. If the load is centered, it is 17 inches back from the hitch pin, so 26.5 inches from the rear bolts. A 500 pound load (including the carrier weight) would make F= 2944 and R= 3444, so the load would have to be reduced to 70% = 350 pounds to keep F within spec.
There are other factors to be considered as well. A greater load might perfectly OK as long as the road is smooth. Any load placed farther back than the normal ball position could affect handling.
