Yes I agree it's the drain for the fresh water storage tank. The one you fill with a hose. Should be a cap on the rear to take off when filling the fresh water storage tank. This is for when you are dry camping and are using the water pump. A little tip on filling the fresh water storage tank. Take an old water hose. Cut off the hose about 15-20 inches from the female end. Then you can attach that to the hose when filling the tank and be able to push the 15-20 inch hose down into the fill spout.