Using a lower pressure regulator may help if your water pump’s check valve is weak or dirty but the problem isn’t the regulator.
There are a couple of easy fixes for city water filling your fresh water tank.
1. Clean or rebuild or replace the water pump’s check valve.
2. Add a new external check valve on the inlet side of your pump.
I would guess that even with the 45 psi regulator your water pump’s check valve will eventually “leak” water into your fresh water tank.
I had this issue and first cleaned the check valve, about a 10-min job. When it happened again a few months later I rebuilt the valve and it’s been fine ever since. It’s a super simple job.
One other thing, I have an adjustable water pressure regulator and I keep it at 60 psi.
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