What size van is it and what watt radiator?
Mine is a fiat scudo so probably smaller, but a large plug in radiator keeps the frost of mine nicely. I dont have it on frost setting but high. On a timer to come on at midnight for a couple of hours and throughout night till morning.
If your having problems, do you drain your hoses, especially pole hose after finishing last job? Fetch your main hose inside and throw an old blanket/duvet over pump etc.
Alternatively, have some sort of hot water system perhaps.......element or gas heater etc.