My wife and I own a large (37 ft) Class A motor home and the Black Tank has never registered empty. We bought the coach when it was four years old and had probably sat unused for an extended time.
The problem with the sensors not reading correctly is that the "master" toilet is a Thetford electronic toilet which will not flush if the black tank is registering Full.
About a month ago, after spending about $300 at a dealer trying to get the black tank emptied, I ran the black tank flush system multiple times with the water running clear when I emptied it. I then filled it again with clean water and added enzyme chemicals and it has been left standing for about a month. Is there any problem with leaving it filled with clean water and chemicals? Thanks.
We always add some Happy Camper and 5 gallons of water after we flush the black tank.
If we don't go anywhere it doesn't hurt the tank and the valve seals don't dry out.
We keep some water in the toilet and sink traps as well.
Yesterday, I put some “Tank Blaster” in the Black tank thru the half bath toilet. I’ll let it sit for awhile, then drive it to slosh the liquid around, then dump the tank to see what the level reads. Thanks for the inputs!