something doesn't sound right here... To my knowledge they would never connect a sink to the Black tank... Those should be toilets only!
I don't know the holding tank layout, but my guess is that your sensor for Grey 2 is not functioning..
I don't' understand how running water in the sink caused the toilet and sink to overflow.... Even if we assume they are both connected to the same tank, shouldn't the toilet overflow long before the sink (sink is easily a foot higher than the toilet)... I am baffled how that is possible so I will shut up and hopefully someone with this model can expound on how the tanks are connected.
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