I think they make some kind of engine heater…I think I installed one once on a toyota (ex-toyota mechanic) probably just a dipstick heater…that just connects to your battery. I would think that this would be fine. I have had friends that have ran their radios in theirs diesels (both dodge and ford) for many many hours with the engine off without any issues with discharging the batteries too much. So I think that would work for you if you can find one. Does you van run for long periods of time during the day to where the batteries can properly charge?