Both payload and towing have maximums depending on van model, but the maximums can't be taken at one time. That is, they can't be combined.
Maximum payload is Gross Vehicle Weight Rating (GVWR) minus van weight. For the ProMaster 3500 model, GVWR is 9350 pounds. So you can carry up to around 4000 pounds or thereabouts, depending on your van options.
In real world the maximum towing will likely be limited by Gross Combined Weight Rating (GCWR). I believe the numbers are 11,500 pounds for gasoline and 12,500 pounds for diesel models.
If the van is empty, you can go up to maximum trailer as advertised. However, if your van carries a typical load, like say tools or RV conversion, then the van's LOADED weight must be subtracted from GCWR.
For instance, if you have a gasoline ProMaster that weighs 8,000 pounds, then the max trailer is 3,500 pounds.
11,500 GCWR - 8,000 pounds = 3,500 pound trailer.
If you have a specific idea of what you are looking at, perhaps we can get a better answer.