I run a bass rigged Starcraft Mariner 180, with a 90 hp Evinrude, 2 large batteries and 17 gallon gas tank. Fully loaded on trailer gassed up and all fishing gear its about 2,000 lbs. Although I tow it mostly with 350 Cu inch suburban, I have also towed it with a 4.3 6 cylinder Jimmy, and a 6.2 Diesel Blazer, All are equipped with a class III frame mounted hitch. I have had this boat for almost 30 years and am familiar with its handling characteristics. I way prefer the Suburban for hauling this rig, especially in the hills. And the Suburban is rated to 8500 lbs. With the Suburban, I have also towed a horse trailer with two horses in it (total weight about 4,000 lbs) but thought that was about max capacity. I wouldn't even try that with the other two vehicles.
Then there is the consideration of braking. Neither of these trailers has trailer brakes, so the car carries the full load. You can go up on your trailer weight if the trailer is brake-equipped.
Then there is the issue of transmission overheat when towing loads, addressed with an oversized tranny cooler.
Bottom line, with over 40 years towing tinnies (as well as bigger trailers up to 18 wheeler rigs) under my belt: in my mind - safety and comfort considering - a rough rule of thumb is to take the rated towing capacity and halve it for the actual weight of what you are going to tow. YMMV
So will your van pull a boat rig that weighs about half its rated towing capacity? Probably, but you'll want to be patient on hills and overtaking, etc. IMO you are getting close to safe max. Hope this helps.