Ah, well the only problem I have had with the monitor is the black reading becoming useless after the first week or two. I don't understand why a better system can't be installed on these things.
Sorry to hear that your black tank monitor is useless for you. We had the same feeling with each of the several RV's we've owned prior to our PC, but have had no major troubles with the black monitor on our 2014 2552 after more than a year of using it. Only in a couple of cases did the black sensor not show empty after fully completing the entire routine that we list below.
There are a number of posts on this forum with ideas to keep the black tank monitor operating correctly (some by Sparky, for example).
Here is the routine that we use:
We dump the black tank about once a week during an extended trip (flushing it with a full tank of gray water each time),
Add a package of Thetford Aqua-Kem Dri and a gallon of so of water to the toilet,
Always do the same as above at the end of each trip, and
Back flush the tanks with the built-in flushing system whenever the tank monitors don't show empty after a dump.
Hope you find a routine that will work for you.
Bruce and Sharon