I thought about using this 12V heater solution for my trailer, but then after some thinking I don't want to kill my batteries just to heat up water that is needed for only a few minutes for a shower or such. I use my trailer for base camping, not for expedition trips, so the battery might not be getting a charge for quite some time. One possible energy saving solution would be to have a smaller 5g tank for the heated water separate from a larger cold water tank. This would use a lot less power to heat up and would be easier to keep hot over a period of time.
The 12v heating element is a great idea, and will be a simple solution for a lot of water heating needs.