There is another option, a 12V to 12V DC charger. A good one will cost more than an isolator. The charger is powered by the starting battery. A quality charger will provide 3 stage charging to your auxiliary batteries and act as an isolator. It will also have settings for the type of batteries to be charged so the starting battery can be different from the auxiliary batteries. I believe the Ford Transit comes with an AGM starting battery, being that it is located in the cab, under the driver's seat.