About a week ago there was a post, (see here) about bugs in the black tank, sounds like he had/has quite a problem. Two summers ago we had our first work camping experience, and after reading posts on here & other forums about leaving both grey tanks open, and the black closed, I left the gray tanks open & let them drain at will. Before we got there our sensors for the tanks worked pretty good. When we left & to date, both gray tank sensors don't work. They'll show anywhere from full to 1/3 full when empty. I'm kind of thinking that the residue inside the tanks dried & now I can't seem to get them to work all the time. I've tried the Geo Method, & everything else I can think of. I can get them to where they'll show 1/3 for a while, but it doesn't last. Now I've given up & when the toilet burps I dump all 3. The black tank will work "most" of the time, and when I give it a good flushing, that sensor will work for a while.
Also, when I flush out the black tank, I'll use the built in flush system, I'll let it run while dumping the tank & leave it running for a while after it's empty, then close the valve let it fill for a while, then dump it. I'll do this several times, leaving the flush system running all the time, until the water coming out is clear. It's amazing that even when the water coming out is clear, there is still "stuff" coming out. Also while I'm filling the black tank to get a good flush I'm careful not to fill it too full, so it doesn't overflow through the toilet & into the bathroom.