oil filled radiator every cold night, works perfect, add a small fan to move the warm air around so the heat reaches every corner of the van.
took the van to hydramaster the other week for a service and the forecast was minus and i was staying for the weekend. they filled with antifreeze and the van stood on the sea front at minus 7

, drove home, emptied the anti freeze out and it was all ok. have to admit i was a bit wary but would do it again if leaving it in cold weather,
colin
ps also released the pipe from the demand pump to empty that of water.