The hot water tank fills up either when you hook up an exterior water supply to the inlet on the exterior of your trailer or when you fill your fresh water tank and then pressurize the system by turning on the water pump.
To drain your hot water tank you open the exterior access door. Open the pressure relief valve and then remove the drain plug.
'09 F-350 6.4l Crew Cab
'10 Komfort Trailblazer T262BSLE