A lot of people use the GEO method. The original recipe used bleach but most of what I see now don't use it. The amounts vary but it is just water softener and dish soap. All I have ever done is put a box of baking soda into the toilet and flush.
The following is from this site
http://wheelingit.us/2012/03/21/rv-tank ... eo-method/The GEO method is a home-made cleaning technique that’s been around in the RV community since (at least) the early 80’s. Where the name came from no-body seems to know, but the original method is HERE. The original formula uses chlorine, but that can ruin your valves so these days most RVers use a slightly modified method with just 2 common items, both of which you can get at any big department store:
Calgon Water Softener -> This stuff helps to break down water surface tension, clean scum and prevent hard mineral deposits. I’ve used Borax on occasion when I can’t find Calgon and find it works just as well.
Dawn Dishwasher Detergent -> The “original formula” blue detergent is excellent at cutting grime. Laundry detergent works too.
After you dump pour 1 cup of Calgon and 1 cup detergent into the tanks. Leave them in until your next dump. Before driving some RVers will also throw a bag of ice-cubes down the tank to “scour” their walls (we’ve never done this, but others swear by it).
And that’s really it! We don’t use the GEO method each time we dump, but just whenever our sensors need an extra cleaning. Either way it’s a simple, easy and environmentally-friendly method to keep sensors mostly clean. And in a moving house on wheels that’s really as good as it gets.