I saw a video on this very question. the guy filled his black and his grey tank with a five gallon bucket, until the sensor light just came on, then he kept filling to see how many gallons he could put in before the water came up into the shower. then he knew for sure just how many gallons he could safely put into the tanks. to me it was a waste of water. I for one go until the lights come on then dump. As far as I'm concerned it is full and needs to be dumped ASAP
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