It is a constant challenge to keep the grey & black tanks clean so that the gauges work properly. This is sometimes caused by toilet paper wrapping around or attach to the sensors. Some people have flushed massive amounts of hot water through their tanks, some have used massive amounts of ice, some have used various cleaners such as Dawn and water. One thing that I found that helps is to use septic safe toilet paper as it breaks down easier in the tanks and is less likely to wrap around or attach to the sensors.
Since yours stopped working all together there may be a problem with the sensors themselves or the connections to them. Since they are in the tank, there is no easy way to get to them. You may have to drop the tanks and investigate. Sounds messy to me.
