Thanks for the great advice. The only sticker I found on the van shows a front GAWR of 3,400' and a rear GAWR of 3,800'. The GVWR is 7,000 pounds. According to a Ford payload capacity document, the maximum is 1,920 pounds. I found another comprehensive calculator on changinggears.com. Based on the values I researched for the van and calculated for the estimated load in the van, I think the dry weight of the trailer will already maximize the van's capacity. That doesn't give me any cushion for added performance going up hills, etc.
Sounds like we will have to shop for a new vehicle with more towing capacity.