The probes are in the side of the tank, under the trailer. I don't think you'll be able to get to them, as the 'stuff' on them is probably some toilet paper on the inside.
You can fill the tank up part way and drive around to slosh the water and clean it off.
Do a search for black water tank recipes, there are a bunch of different things people put down the toilet to help keep it clean.
2014 Silverado 2500 6.6L TD and 4X4
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