Those factory installed probes are worthless.
If I could get to my tanks, I would do the Horst Probe mod but these GT MH's have the tanks up inside the chassis and are sprayed with a foam insulation underneath.
So, I generally flush the black tank well at each emptying and when the probes show it full, I watch the level manually or, if convenient, dump.
The gray tank is harder to watch manually but as long as there is nothing entering the shower, I am OK.
Life is a journey, not a destination.
2005 Georgetown 359ts
Bill & Virginia, Kentucky
Nights camped in 2011...78 , 2012...73,
Nights camped in 2013...123, 2014 ...101