Usually a valve for this is located in the water bay. My friend's Monaco has a panel accessed from the bay next to the water bay where a panel slides to reveal the group of drain valves.
Hope this helps you.
Kent and Sally Lunda 2006 Newmar Mountain Aire 4141 4 Slide 400HP Cummins, Spartan Chassis, Hydro Hot 2004 Honda Odyssey (Toad)
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