I do the Geo method a couple of times a year, once a month when I was on the road. 1 cup blue Dawn, 1 cup Calgon liquid water softener, 2 buckets (about 5 gallons) of water between campgrounds. I'm also not too fussy about whether the probes work or not.I can look down into thew black tank to see where the level is.
Jerry & Jeanne
1996 Safari Sahara 3530 - 'White Tiger'
CAT 3126/Allison 6 speed/Magnum Chassis
2014 Equinox AWD / Blue Ox