Our PC will be 14 years old in a few months. Our original tank sensors continue to work flawlessly. I think it is because at the end of our RVing season, I bleach the gray and black tanks followed by a rinse, to remove foul odors. Odors are a concern because our PC is stored in our garage, located under our bedroom.
I can't say my annual maintenance process will work for everyone, but it's worth a try if your tank sensors are not working properly.
I put 1/3 of a gallon of house-hold bleach in each tank, then fill the gray tank until I see it backing up into the shower, and the black tank until near the toilet valve. I also add a little bleach in the closed toilet, sinks, and shower pan filled with water and bleach everything clean. I need to be careful walking around to minimize sloshing. I then wait 2 hours, drain everything on the driveway (yes the water smells fresh and clean), and repeat with fresh water, let sit another 2 hours, and drain again.
When I am done, our PC smells like brand new, our garage has no sewer gas odor, and the now 14 year old tank sensors work great.