The check valve is most likely in your water pump. It is used among other things, to prevent city water feed from coming backwards through the pump into your fresh water tank. Some folks say you can try to fix this by running the water pump for a few minutes to try to clear any obstruction that may be responsible for the antibackflow valve from not working. Then try your city water feed again and see if it is still flowing back into your fresh water tank. One cheap way to fix this (or use it as a backup insurance), rather than replacing the pump, is to buy and install a standalone anti-backflow valve from your local RV place or on line. Just plumb it into the fresh water feed line to your pump.
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