Tongue Weight is the weight of the front of the trailer that is resting on the tow vehicle's hitch. The trailer is NOT balanced, but is deliberately unbalanced to have 10-12-15% of its total weight on the front (this helps reduce sway significantly).
So if a trailer weighs 3,000 lbs, from 300 to 450 lbs of that will be on the tow vehicle hitch and 2,700 to 2,550 will be on the trailer axle
One starts with a bunch of estimates of the various weights, but one should end with actually weighing
everything, fully loaded and fueled and provisioned for the road, on a set of scales.
The towing capacity for your van should be in the Owner's Manual under trailer towing.