If the van is overheating and the coolant is low, you need to add coolant. If it's still low, you have a leak. I'd probably take it to a trusted mechanic and say that you believe you have a coolant leak and then let them fix it. It can leak from a hose or the water pump or the neck or the radiator or a bunch of other places. Worse, when you have one small leak, you often have several other small leaks. Worse still, a small leak can rapidly turn into a giant leak and strand you or destroy your car.