The tank monitors do not work, never have, save for a freshly steam cleaned tank, and never will unless you steam clean teh tank.. Poor design. You do however have choices
1: Seriously clean. There is one cleaning wand (I made my own before I saw it) the Valterra Master Blaster.. I do not like the design of it but mine is different. a 1/2 inch pvc pipe capped with a hole about 1/4inch. in the SIDE of the cap/pipe (not the end) blast that ... stuff off the wall so teh sensor can dry out.
2: Horst Miracle probes.. I plan to test them... Some day.. But have yet to get the reound-tuit needed Company claims they work I've heard good things from alleged customers but I have not tried.
2: See-level that works. Totaly different type of sensor.