If you get a 8 litre lpg heater fitted, you can preheat the water to a reasonable level, which then acts as a radiator in the van, job done.
I tend to preheat 100 litres over night, then fill in the morning, that tends to be the optimum time for preheating, maybe 15 min
The water doesn't have to be really hot, just warm enough to hold the temp above freezing through the night.
Hope that helps!
Jon