I tried the parafin heater,.. the stink nearly killed me, and the one night the wick was set slightly wrong the whole place got covered in very dirty and hard to remove soot.
I'd recommend a LPG water heater,.. heat your tank to a lukewarm temp, 20-25 deg C will do, and it'll act like a giant radiator all night and keep the van frost free.
If you can park near the house and a trailing wire wouldn't be a hazard, any cheap oil filled rad with a frost protect function will do the job too.