It takes ages to defreeze a tank full of water, my goodness. I use my van for personal things now and then and the sound of the ice in the tank is, well, I dunno, a bit scary I would say. I had visions of the ice piercing the tank.
I do have a greenhouse 180w heater in there, but it doesn't even remotely get warm in my van. Must be because it's got almost no insulation? Citroen Berlingo, old model. I closed all the vents, let's see if that helps.