There are physical situations that cause these tank indicators to display incorrectly. Both our black tank and grey tank has incorrectly read at times. I assume it has been caused by some kind of "debris" in the tank sticking to or interfering with the tank level monitor. The last time this happened with the black tank, it read incorrectly for several months, in spite of my making several attempts to correct through various cleaning methods. Finally, I tried something different, just based upon a hunch. After completing a black tank flush, I added our usual chemical, and I also added one cup of borax powder, and about a cup of laundry detergent. In a few days, after the normal black water tank flushing and backwashing, the tank indicator read correctly for the first time in several months.
In my "opinion", it was the BORAX that made the difference, because I had tried detergent in prior attempts, and that didn't do the job. So, now, after flushing the black tank, I just add a cup of BORAX, along with our tank chemical to help keep the tank level monitors clean.
So far, in the 8 to 9 months we have been using the BORAX, we haven't had any more problems.
