Best option is buy a Thermopure or similair heated system. You can recirculate the water back into the tank for a half hour or so on your way home or while you're filling. The hot water in the tank will radiate heat for hours and keep the van frost free.
Thermopure is an expensive option but has many other benefits.
seems to be a very expensive way just to heat your van up.
I agree windowwashers, but I wanted a hot system for other reasons as well. The frost protection was a fringe benefit.
Even the thermopure is a pain sometimes as I am driving home with an empty tank and can't be bothered going out again later to turn it off when the preheating is finished. So, sometimes I still use an electric heater. In the early days I couldn't park near my house as I was on a terrace. Tried the greenhouse paraffin heater but I had trouble sleeping and there was an issue with my van insurance. Not allowed to have a naked flame inside the vehicle. So I calle dIonic and they sadi to preheat the tank. Worked very well.
I have tried other options such a lagging with a duvet as I am always trying to save on Diesel. Electric heater with a thermostat is the most convenient but is very expensive to run.
I am now looking at ways to drain the system.