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 ...
They flat don't work, the readings are unreliable. Just pretend it's like a broken gas guage. In mine it's 2 people heavy use 7 days, lite use 10 days, yours will be differant. Add more people do the math. If teenage girls are involved these calculations go out the window.
The only time I ever did fix one was on a black tank, recipe, 65 lbs ice, big bottle of dawn dish soap, pour into tank drive 100 miles, drain.