So you have 1 black tank and they wired it to Black #2 on the panel? That seems strange.
As for the check valve, an initial step may be to turn off the city water, turn on the water pump and then open all the water faucets - this will pump water through the check valve and may clean out some gunk preventing it from sealing properly. Then, drain the fresh water tank, put the city water back on and see if the FW tank continues to fill.
Welcome to the forum!
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2022 Cedar Creek Champagne 38EL - "Big Bertha"
2019 GMC Sierra 3500 Denali HD 6.6L Duramax TD, CC, 4WD, DRW - "Mr. Beefy"
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