I have a 2014 Itasca sunstar with 2 black tanks. One started showing 3/4 full for no reason. I emptied and ran the flush for several minutes and then it showed empty as it should, now after 3-4 hours ...
I have filled my tanks with water and then added a new product called Rid X for motorhomes to the tanks and let them sit for week. I did about a triple dose And it seemed to really help with the sensors. Just an FYI and I got it on the net