Don't see why more people don't fit an indirect twin coil calorifer tank using a hot feed from the van's radiator feed through the calorifier and back into bottom of radiator and make sure you set the thermostat on the calorifier to your desired temperature. Then run a circulation pump to mix it around your WFP tank for the day. Calorifiers with a built in 2/ 1.5 or 1kw electric immersion heater also fitted can be used to quickly heat up set water to a set temperature very quickly if not driving much distance.
Calorifier tanks hold 80% of their heat for up to 24hrs and if you insulate your WFP tank very well you can run this suitably well using hot water with free heat from van's radiator and top up using electric if need be. It works out very cost effective. Only recommendation I would say is don't use Shureflow pumps as they're not robust enough with really hot water. Flojet pumps are noisier but much better with hot water.
I'm really surprised no big companies have got switched on and started producing double skinned, well insulated water storage tanks for WFP use and combined the calorifier method into their tanks. Cheapest heat going even if using electric heater elements as a top up source of heat and safe if fitted safe and allow for pressure relief valve, etc.