Hydroincs heaters are multi fuel multi function devices, they can burn Diesel, or other fuels (electric and in some cases propane) Propane is still sued for cooking by the way.
When driving the engine heat is run through a heat exchanger to provide hot water thus you arrive with HOT water in the water heater.. Diesel or electric (Usually) can be burned to heat the water on site,, When you are ready to leave a pump circulates the coolant so now the hot water heats the engine coolant, which becomes a block heater, thus reducing emmissions and making it easier to start even if the outside temps are... COLD!!!!!!.
Basically I would recommend such a system
Aquahot is a brand name for a specific Hydronic system. Like Band-Aid is a brand name for a self-aheadsive bandage... Or Kleen-X for facial tissue..