[rdlamb]

In a previous post, which I cannot locate, I related what my plumber friend found with his optical snake that he graciously used to scope my commode.

In my TM, the GS factory plumbed the toilet to the blackwater tank so close to the side of the tank that "stuff" would slime down the wall to the tank's floor. Also, the sensors are right there, where the "stuff" will pile.
With this information, I agree, having the bathroom sink drain into the tank would be a help. However, the sink does not connect to the toilet's pipe, but enters the black tank at a different point, toward the outlet. So, water will sort of float up around the pile of "stuff", but does not help wash the pile into a pool of liquid.

To help, I have found that a big container of "dishwasher" tablets, with several of these tossed into the tank when we are moving to another site, will clean the sensors so that I can get some idea of how much "stuff" is in the tank.
Otherwise, it is a guess.