Doing it that way Matt would then mean you would have to delay transfering it to your van tank until the morning prior to using it as otherwise it would cool down overnight. Also the reason I think its better to heat it in the van is that when its transfered, for me anyway, it gets a chance to cool down (I have to pump my water 40m from the tank to the van) and if you have the heater in the van you can at least reheat the water back to what you want it to be before setting off.
Not saying its not a good way, depends on your personal circumstances.
My 3KW heater will be here by the end of the week, looking forward to that, that'll put some sparks into the water
Matt