Electric fresh water dump

That solenoid valve is a diaphragm type valve. It has an orifice and requires a pressure differential to operate. The label on the picture appears to be generic, it does not have information required. If it opened at all, it would take forever to drain, and I am pretty sure the pressure differential would soon not be enough to keep it open and it would close before the tank was empty.

I would use hose and relocate the valve to a ball valve that was accessible. The bigger the water drain, the better.