Fresh water tank anti-siphon
Without seeing or knowing about your situation, I have made a very generic sketch of a typical fresh water tank overflow.
1. Notice that the overflow tube enters the tank at the top. Depending on how talented and how hurried the installer is, the length of the overflow into the tank and therefore into the surface of the water really matters.
2. There is most likely a duckbill valve under the RV floor, which is there to allow water out, but nothing in. Somewhat like a check valve, but mostly for dirt and bugs. That item should stay in place.
3. What many, many owners have done is illustrated on the right. When it is accessible, the elbow is removed, a tee put in it's place, a tall standpipe is added and the end is capped with a venting cap. Some have used just a hole, others have attached a tube. The whole point of the vent hole is to break the suction of the water as is sloshes out of the tank and then down the pipe. The downward flow is creating a suction that continues to draw water until the water level is below the overflow pipe in in the tank even after the sloshing has stopped. The added stand pipe should be as tall as possible to allow some sloshing up the pipe, but not out the vent hole. Note: the addition of the tee can be in place of either elbow.
Remember: this is the fresh water tank. If you are drinking or cooking with this water, protect it in what you do.
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