I never bother with the sensors anymore. After 4 showers or so the grey tank needs to be dumped. If I need a little more grey capacity I will close the outlet with a 3rd gate valve and open both tanks and let the additional grey water drain into the black tank.
In short I never check the gauges I don't know if they are accurate or not because I just don't use them any more.
If the grey gets full the shower drain will back up. If the black gets full you'll know the toilet simply won't flush, however I have never camped long enough to fill my black tank. 4 of us can easily go a week without filling the black tank