We just purchased our 2 nd older RV, a 1988 Coachmen 5th wheel. I was cleaning the fresh water tank with diluted bleach and after a day went to flush the system by turning on the pump and opening the faucets. I heard the pump start as I flicked the switch and opened the faucets. After a dribble nothing came out. I checked the pump and it was working properly but in order to do so I had to drain the tank. When I opened the valve nothing came out of the drain. After removing the pump I noticed there was water in the outflow side of the pump but not a drop in the intake side. Assuming no there was a block in the lines I used an air compressor to blow air through the intake hose and somehow, with a tank holding at least 18 gallons of water the only thing that came out of the drain was air! I tried it the other way blowing air into the drain and sure enough felt air through the intake hose. How is this possible and what can I do to fix my fresh water system. Bear in mind the fifth wheel is parked in the woods (we live on 40 acres of woods) and there is not access to city water or electricity. I used Gerry cans to fill the tank and a generator to run the air compressor. Also the previous owner said he had drained the tank just a few weeks ago and had no issues.
If the fill cap is off, the drain valve is open and not plugged, then if there is water in the tank it will drain out. Gravity does not take days off. Sounds to me like the fresh water tank is empty.