Yeah Stan, it was the black sensor. They and the gray are the most likely to get fouled because of the gunk that can dry inside the sensor's port.
I understand that it's possible to get at the water tank's sensor by removing a panel in the back, from under the coach, but it may be no easy job. Seems I've read some argue that the panel isn't removable though; I don't recall the Forum thread exactly, and it may vary with model of coach.
Just not sure why, though, a sensor in a tank of clean water would fail, unless like our black tank when new there is a chunk of tank, from the installation drillout process, stuck in the sensor. If removing the panel can be accomplished, it may be just the data connector is corroded or loosened on the sensor back. Or check the matching data connector (p/n 16617089) at the Tank Interface Module in the waste tank bay. Otherwise at that point, with panel out of the way, unscrewing the sensor and cleaning or replacing it should be as easy as Larry notes.
Regardless, I'm glad your SeeLevel is doing the job for you now. This thread may provide others with fodder to chew on.
Joel