Turn the pump on and let it pump water from the tank. Then get down on the floor and listen for the pump noise. Start in the area of the fresh water fill location.
That's how I found the one in neighbours 5th wheel. Hers was hidden in a small side table between the two lounge chairs in the living room, no where near any of the other plumbing.
Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.