That valve is normally next to your fresh water tank....probably located under your bed. Look for a line coming out of the tank that may go into a two way split with one side going to your pumo and the other going down, through the floor. Open the valve on the one going down through the floor to drain the tank. Since there is not a lot of standardization in the builds, the drain line may come out of the tank and go directly to the drain valve and down to the ground, without any other line attached.
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