I used a tank connected to my van radiator system for 10yrs it contained a stainless steel coil inside a 100gl water tank, it work brilliantly, the water got to Temp that you could not put your hand in the tank. The tank was insulated and had a heating element fitted so you could preheat the water over night Then the van kept it hot throughout the day.
The Tank came from a system available in the USA made by BaneClene which made carpet cleaning truckmounts
I no longer use the tank if anyone wants to buy it😀