A gas on demand heater like the "fogwash" is brilliant in the winter. Turn it down low to avoid thermal shock,.. but you can work away down to -6 or -7 without pipes freezing.
Obviously its not clever to be spraying water over footpaths at those temps, so it try to do work where I know there are flowerbeds under the windows (and trad doorways) until it warms up a bit - just use common sense & you'll be fine.
If you combine one of those heaters with a frost protect controller then you can protect your van system from freezing too,.. just re-circulate the hot water back to your tank & it'll act like a big radiator.