Do you have this as your fill / divert panel?
Behind this panel is a mass of white hoses. New from factory, my bet is on a miss plumbed connection between the black tank rinse and the fresh water tank fill, the only way the water can get there. It is a tight space and tough to see what goes where, I labeled all of mine just for sanity purposes.
There should be no connection between the black and grey tanks, was a faucet left open?
Manual attached.
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