Load the van up with the kids and similar gear you would travel with, and find a scale. Subtract that from the GCWR (Gross Combined Weight Rating) that is in your manual, and that is what you can tow. It looks like it may be about 10,800 if you have the 3:31 ratio as it says towing of 5400lbs. But that is with the van empty.
Don't be surprised if it is quite a bit under that 4200 lbs dry weight. And a large frontal area trailer hurt capacity too. I think there is a chart on the Ford Towing specs web page about how to figure that.
2015 Transit Van 250 MR 148" EB 3.31LS rear AC. Born on 12/18/2014
Last edited by star gazer; 11-16-2016 at 08:12:PM.