[rdlamb]

Dan:
My TM is only 2 seasons old, but I have learned to live with the black water tank sensor being incorrect.
Some things I have learned:

A bag of cubed ice and a large bottle of Pine Sol, prior to a move will slosh around and help clean the tank. The trouble is the Pine Sol will foam, which will leave a film over the sensors. Ergo, I follow up with a thorough water wand spray afterwards. Sensors are still not reading correctly, but usually the black water is now reading only 1/3 full.

I have now switched to a couple of dishwasher tablets to scrub the tank, these do so without foaming. But, I am assuming that the water spot treatment in the tablets is keeping a film from developing on the sensors. JMHO, nothing scientific.

Keep a good chemical treatment in the tank.

Finally, I have a plumber friend that now uses a fiber-optic snake to view sewer lines. Someday I'll beer bribe him and see exactly what is in my holding tank. I think there might be a market for this type of service, and will do some stumping to get said friend to consider offering that service.