Trailer Tongue Weight and Sway
The tongue weight should be 7 to 8% of the weight you are towing.
Lowering the tongue weight would probably make the sway problem worse in my opinion. Do you have the same type of hitch? Is the boat trailer level with the Van? Is the new van rated at the same load carrying capacity as the old one.? Is the distance from the rear axel to the hitch the same as on the old van? A longer overhang would exacerbate the sway problem.
All things to consider... A good load leveling hitch will do wonders when you are towing a heavy load.
Just a few thoughts!