Check valve at pump not closing which allows water to back feed into fresh water tank. If you clean it out or change it, it will eventually happen again most likely.
Here's your forever fix. Put a shutoff valve in the line between the tank and the pump.
Keep valve closed when using city water, open to use fresh water tank when not hooked up to city water.
Will cost you $20 or less for fittings and a shutoff valve. Done
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