Regarding your original question. It's a good idea to empty the holding tanks, add a good amount of liquid tank cleaner full with water and let it sit 24 hours. Then flush the tank a few times. This will loosen up the crud that sticks to the side of the tank where the EXTERNAL sensors are located. See drawing. Most coaches use external sensors so you can not harm the sensors or the poly tanks.
John & Debbie
2008 Fleetwood Discovery 40X Full Timers Since Oct 2015