One way that tanks empty...
What happens often with fresh water tanks as large as ours if that the owners hook up their water hose to fill the tank and then walk away and wait for the overflow tubes to start kicking out water to let them know their tank is full. While this may be more convenient than waiting and watching the monitor panel to monitor the water going into the tank, it often results in a "reverse vacuum". What happens is, the tank becomes full, and the water begins to flow out of the overflow tubes, but that overflow is not able to keep up with the amount of water coming into the tank from the hose. The result is, the tank begins to swell. (Note: there have actually been some who have caused their tank to swell so much that it caused damage to the coach).
Typically, a person stops the flow of water into the tank when they notice the overflow system kicking in - however - the tank may have already swollen up which means they can now have the reverse vacuum that I mentioned. It is very similar to cyphoning, in that the pressure now is going to go the other way, creating a vacuum, and emptying the water back out of the tank through the overflow lines. If a cyphone or reverse vacuum situation is created, it can actually keep going until the tank is 2/3rds to half way full before it stops. THAT is why some folks have problems filling their tanks up all the way.
The answer: ALWAYS be sure to keep an eye on your monitor panel when filling up your fresh water tanks. This will dramatically cut down your chances to create the reverse vacuum effect and loose the water that you were waiting so patiently to put in to the coach.
ST