Most of the solids break down to a fair degree, at least in the sense of turning to mush as opposed to being chemically or biologically transformed, simply from being in water for a period of time.
I find my (standard, probe-style) black tank level monitor is not particularly reliable, the gray tank one mostly accurate but not perfectly so, and the fresh water tank one trustworthy. None, of course, are very precise, with extremely limited resolution. I also find that the gray tank gets pretty near full when my fresh water tank gets pretty near empty, and by then there's sufficient stuff to make emptying the black tank reasonable, so I service all three tanks at the same time based on that.