Thanks for the replies. I will try the Geo Method but I have tried cleaning the black tank and the grey tank and the sensor readings have not gotten any better.
If the sensors are corroded, I would either have to replace them; which I can't get at on the black tank, or install outside sensors like the See Level ones.
P.S. I did get dinged for the second post
. Was just looking for more views and suggestions in the general section.