I have found the best way to keep the sensors working good (both black and grey), is to fill with a few gallons of water, enough you know the outlet pipe is full and the bottom of the tank has enough to cover it (5 - 10 gallons depending). Then use some Dawn Dishwashing liquid (a nice generous amount), and then take the rig and drive around so the water in the tanks will slosh around real good, and will splash against those sensors. So, the longer the drive, the better chances the sensors will get pulverized with the water. This cleans them more effective than anything else one can do. Caustic chemicals are not the best solution and usually cost more than they actually deliver (not worth it). But simply keeping a bit clean water and drving around will do the best. Once you reach your destination or home, dump the tank. If you have the access, fill it and dump again.
After a while, you learn, after dumping your tanks, to add some water back for the drive. If you do, your sensors will continue working as they are suppose to. My sensors have always worked. This is so simple to do, just add water and drive on, I don't understand why so many have problems with sensors not working.
Edit:
Of course, if your camper sits permanent, or seasonal and never moves, maybe the better solution would be do some replumbing and remove or bypass the holding tanks. If the camper never moves, I see where the sensors would start failing with "gunk" getting caught on them.