Back in the day when that was built, I think the ford van was THE towing vehicle, and the surburban was the only competitor. Pick ups of the same weight class did not compete, but I am thinking that 5,000 lbs was the towing capacity limit, and that is on a standard van, not a conversion van. Whatever extra weight that was added in converting it, would have to be subtracted from its towing capacity.
You may want to put your van on a scale and compare that to its listed curb weight, to determine how much towing residual you have. Be sure to deduct passenger weights, and any additional gear you plan to pull/ carry.
When at all possible you want to pull the trailer with empty tanks, consider that 40 gallons of water weighs approx 400 pounds.
You may want to consider that your van's curb weight is calculated with all fluids full so to add carrying capacity, run with half a tank that should give you an extra 70 pounds or so.
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BroncoJoe19
2006 Alt Fuel Jeep (electric)
'98 windstar 3.8L
'99 Crown Vic 4.6L
'90 Full Size Bronco 5.0L
I'm not a professional mechanic, take my suggestions with a grain of salt, or a cup of coffee 
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