The black tank sensors get dirty easily and are less accurate than the grey sensors in my experience. If you can see the black tank level looking into the toilet, that is the most accurate. My trailer has 2 each 32 gal. grey tanks and a 32 gal. black tank. The rear grey tank which receives the shower water fills the fastest when the wife & I are boondocking. We both are very conserving of water however we only have 32 gal. of freshwater so that is the limiting factor for us. The sensor electrodes are never at the top of the tank so you always have a bit more capacity when the full light is on; seems like a reasonable safety factor to me.
