Those 'sensors' are just a metal probe stuck thru the side of the tanks.
4 probes......
1 ground (down low)
1 for 1/3 level (down low same level as ground probe)
1 for 2/3 level
1 for full
They work off conductivity.
When 'wet' a circuit is made between the level probe and ground probe
ANYTHING on them (Stuff, toilet paper, scum etc.) will cause them to register.
You can buy probe cleaners, you can back flush until blue in face, you can use the GEO Method......you can end up pulling your hair out.
K.I.S.S.
When toilet 'burps'........time to dump black tank
When a sink or the shower starts to drain slower then usual...time to dump grey.