I really doubt that you are transferring the full rated weight of the bars in hitch weight. Those numbers are basically for reference, i.e. the 1500 lb bars can handle more weight than the 1000, etc. The only way you can know for sure how much weight you are transferring is to get actual axle weights from your van and trailer. (since trailer axle weight will increase with WD bars engaged)
A few years ago I helped set up a Ford F550 rescue truck towing a 7000 lb trailer with a 900 pound tongue weight. The truck was slightly over rear GAWR so we used a 750 lb WD hitch. We were only able to transfer about 300 pounds off the rear axle, which was just enough. IIRC about another 200 pounds went to the trailer axle. These numbers will vary with the distance between the hitch ball and both truck and trailer axles, and possibly other factors. The only way to know for sure is to weigh it.
