If you want the black tank & sensors super clean, use some Liquid Drano. It won't hurt metal or plastic. Our owners manual says to use it. We've been using it once or twice a year for 5 seasons. Only problem is, the sensors stop working again in no time.
Resistance is futile. Trying to get the tank sensors to read properly is a waste of time. Looking down the toilet is a better method. I installed a shutoff valve at the back of the toilet so I can look down and see the level of the "stuff" more clearly.
We installed the Horst Miracle probes. They didn't help at all. I think the problem is because the in-tank rinser is on the same wall as the sensors and they never get rinsed off. We also installed them in a previous TT and they worked great. The size, location of outlet pipe and sensors of your tank will affect how well the Horst probes work.