Why does water pour out of the bottom spout on my RV's water heater?

I just bought my first RV (1978 with an American Appliance Mfg propane water heater model 6 ARV) and whenever the water system is pressurized (by the 12v pump or when hooked up to city water) the water flows right through the hot water tank and out of the spout at the bottom of the heater (it looks like a garden hose will connect to it and inside has about 1/4" hole). The only way I can stop it so I can use the water in the trailer is to turn the water off completely for the water heater. Should this spout simply have a plug screwed on to stop the water from flowing out or could there be something else going on? My RV dealer told me that was a spout for the anode rod but the Anode rod is actually at the back of the tank with access from inside the trailer, not outside behind the water heater door. The spout in question is located near the bottom of the unit, on the wall behind the gas/air tube. Anyone know anything about these heaters that might know?