Generally they run with an electric heater and water from the closed loop of "freshwater" coolant from the engine. Open up the engine compartment and the area where your heater lives. See if there are hose connections from neat the thermostat and the freshwater coolant engine pump, and follow the hoses to the heater. If you have a freshwater cooled engine, with a heat exchanger, that is. Look at the heater. Does it have electric wires going to it? Do you have a hot water heater breaker on your electrical panel? There should be a drain at bottom. Open it to see if there's water inside.