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.
2007 Keystone Challenger 5th wheel, Ford F350 Super Duty 6.7L Diesel, Short Bed, 2 dogs and the cat and rolling down the road full time since May 2014