I've had several trailers, and none of the stupid sensors every worked as advertised. I back flush and have a tornado flusher, and it simply doesn't matter. By design, they are stupid.
Let's stick a piece of screw in a tank filled with solids and tissue and see if anything will hang up on it??? That's poor engineering. Of course,******is gonna hang up on it. If a piece of tissue sticks to the top sensor (full) and dries on there, you're done in. Unless you fill the tank to the very top and let it soak forever, that paper is likely never coming off.
With all the brainiacs in the engineering world, it seems one would fine a more reliable yet affordable solution. We just have learned from experience that we need to dump about every five days.
We can actually sort of tell how full the black tank is by the sound of the water when we flush. It changes pitch as it gets full.