Lift the mattress on the passenger side, you will see an access panel you can lift and slide to the side. With a flashlight look inside and you will see the water tank. Coming out the back of the tank, on the bottom, out will see a short of white PVC pipe and a few inches from the tank, on that pipe, you will see a 1/2 turn valve where the handle will be set so it "crosses" the pipe. Give it a half turn so the handle is parallel to the pipe.
That will drain the tank.
You may have to move the heater duct (silver foil duct) a little one way or the other to see the valve.
