I also use the Geo method along with rinsing every now and then and have even added a bag of ice at the gas station occasionally to knock off some of the crud. The ice will help, but only for a bit of time before the sensor was misreading again.
I think the only useful solution seems to be to upgrade the sensors (which I haven't done), or just learn to live with it. I've always been able to estimate the gray water tank and can just look down the bowl to see the black so not really motivated to do that mod (yet).
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD