Here's something on that subject:
Having said that, this is our third 5th wheel and we have just given up completely on all the sensors. We have a pretty good idea of how long we can go with our tanks. The black seems to go a long time, and that would be because there is not a lot of water going in there. You can get very stressed out over that sort of thing, or you can just live without sensors.
On another note, it is not a good idea to totally fill up your black tank with water to clean it out. My husband has done this in the past with not very good results a couple of times. Now he no longer does that. We have a clean-out thingy for the black tank - you put the hose to it.