Lisa, depends what van you have as to what position the switch needs to be in when towing to charge the battery from the car.
However, if your battery has gone flat over the winter lay up, then there will not be sufficient charge from the car to charge it up properly.
It is possible that by leaving the battery in the van , and discharged for a long period that you have done irreversible damage to it, and it will never charge again.
Batteries should be removed from the van if overwintering without use, and be kept fully charged at home, in a cool place, but not subjected to frost.
Your best option now is to try a ful charge at home with a multi level charger, as are available in most places now, and see if you can getit to charge up.