just put a popane heater in the back of your van and leave it on all day try to keep door covered with a blanket etc and keep water runing when your out cleaning i'v been doing this for 3 yrs now and have worked in -5 with no probs also i have a water temp in the back of van and it gets up to 15c ( i dont have water heater ) my work shop heater i got from ebay for £ 40 good luck