With no responses on the forum, I tracked it down myself with the help of a borescope. The black tank is underneath the toilet and you can access one corner of it under the aluminum cover next to the toilet over the vent pipe. The Seelevel sensor is attached to the front wall of the black tank, pretty much in line with the toilet. The two wires from the sensor pad are joined with wire nuts above the tank and the wires continue into a harness.
All my wires looked secure, Although it's a bit tight to get fingers in this space. Replacement of the black tank sensor looks just possible with the removal of the toilet and its support, but again space is tight. Not an item designed for easy maintenance, I'm afraid!