Just so you know the ACE 30.4 doesn’t have an Anderson valve. So eliminate that quick. It’s a bad check valve on the pump. There is an isolation valve between the FW tank and the pump. Close it and the city water won’t back feed into the FW tank. I know that doesn’t solve the check valve issue but your tank won’t fill up and overflow. Check valves in pumps this small aren’t usually external so you’ll need to get a new pump.
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