Gray water tank sensors tend to get mucked up with grease - I have found that Dawn seems to be the easiest way to tackle those sensors.
Black water tank sensors tend to get much up with toilet paper and other "stuff" ... I use a "butt wand" sold at any RV outlet which you put down the toilet and use a high pressure spray to clean off the sensors. If your patient you might consider adding a bottle of septic tank cleaner to the black water tank ... fill to the brim and wait a week --- tends to dissolve everything. Many RVers tend to ignore the black water tank sensors - you kinda know when that's getting full .... gray water tank is more important in my book.