Each sensor/tank setup varies in accuracy. You really don't know empty until empty and Full can mean almost full or OMG! The reality is that are all an approximations.
After a few times, you will figure out what your gauges mean. Our black tank signals getting full also by sound as we flush. Last RV I could see the level in the black tank when flushing as it got high............... ugh. Depends upon the plumbing.
Some tanks are visible - I can see our fresh water tank level through opaque tank sides in a bay.
Bottom line - the sensors are guesstimates. Sensors inside the waste tanks will get a little debris on them and cease operating correctly. Most of us quit worrying about and figure out when we are approaching empty or full. Happy Camping!