Water tank gauge

If you're gettings readings all over the place it's likely that there is something, like toilet paper in the black tank, or a film of grease in the grey tank causing it by covering the probes. You can buy several styles of spray systems that are directed at washing off those contaminates or you can replace the OEM probes with Horst Miracle Probes. I've only read reviews for these and it seems they work OK.

I just go by the grey tank readings in my rig. When it shows 3/4 full, I dump both tanks. Can't trust the black tank readings.