Factory installed tank sensor are cheap, unreliable and essentially worthless. Black tank sensor is the worst because of the amount of solids that seem to be the culprit. Gray tank are the same sensors but normally experience less issues.
There are aftermarket sensors available but require access to the tanks to install. So, here is an easy try - Dawn
dishwashing liquid. It can help both the black and grey tanks performance by removing grease and soap scrum.