When to dump?

Black tanks, if dumped when they are too low, will build up a mound of stuff near the drain pipe that will end up drying rock solid, with no good way to get rid of it.  That will allow further stuff to build up until you have major problems.  

There are 3 rules to prevent problems with the black tank.

Rule #1, after adding chemicals via the toilet after dumping, add 6 full toilet bowls of water.  That ensures that anything that may be left behind will stay wet.

Rule #2, always fill the toilet bowl at least 2/3 full for any #2 trips to the bathroom.  You may need to flush twice, depending. (there is a reason this is rule #2, lol).

Rule #3, never dump the black tank until it is at least 2/3 full.  Since the sensors are almost always useless sometimes the best you can do is wait until you see the pipe below the toilet start to fill up (at that point you usually have one more flush before it is full).  Then you know it is time.