Your water valves and Black tank flush are two separate systems, water from the hose can only go in thru the flush. Make sure your dump valve is open
OK, that confirms my thoughts. At first it sounded like the FW tank was filling but I guess it was the water working its way through the plumbing. It took a minute but eventually water came out the dump valve.
Thanks.
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2017 Dynamax REV 24RB
2018 Ford F-150