Gray and Black tank sensor issue

Tank sensors DO WORK. BUT, they fail because RV'ers do NOT follow normal procedures to keep the sensors/wall clean. LOTS of Rv'ers do not use Black tank chemical treatment. THAT causes Black tank issues. MORE THAN LOTS do NOT use Gray tank treatment. THAT causes Gray tank issues. Once the metal in the tank probes become coated and the sides of the tank become coated, they will then NOT work correctly. No amount of cleaning will restore or remove that caked on slime on the side of the tanks. The pressure wash usually will get rid of some of the wall slime. The system is simple. You have 4 or 5 probes. 1/3-2/3-full or 1/4-1/2-3/4-full with a single ground probe. When the liquid touches the probes this completes the circuit and turns on the appropriate light on the panel. I ALWAYS ask customers that complain, how come the FRESH system ALWAYS works? Because there is NO dirty water. I have LOTS of full timers that never have a problem because they spend the money and time to treat the 2 tanks from new. It always seems the complaints on these forums always come from newbies that buy used and the previous owners caused the problem. LAST, yes, external sending units like the See Level system is a good way to go to prevent false readings. Doug