Two things cause this. One is a faulty Anderson valve and the other is a bad water pump check valve. First time in my now former fifth wheel was the Anderson valve. The second time it was the water pump check valve. The fist time I removed the Anderson valve center stem. One of the seals was crumpled up and out of position. After that I took it apart once a year and lubed it with plumbers grease. Worked smoothly after that. The second time I was on a trip and took the stem out of the Anderson valve and found that it was fine. So I purchased an external check valve as a quick fix and put it in line with the water pump. Fixed the problem. I do not know any other reason the fresh water tank would fill except for these two failures. Those without an Anderson valve would not under stand and those with a separate external not integrated fresh water tank fill would not as well.
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