The single wire runs through the bulkhead and UNDER the water tank, which is mounted semi-permanently with a 1X3 wooden frame:
There's not much room but I managed to wedge my hand down between the hull and the tank to...wait for it...knock on another hidden tank underneath the water tank! In the above picture you can see two hoses at the bottom of the frame coming up from underneath the water tank. These two connect as one and run aft all the way to the transom where it exhausts out a small clam shell vent.
Now, taking another look at the hoses attached to the head we see a third hose (the one in the middle) that runs along the sole through a hole in the bulkhead where it disappears under the water tank: