Welcome to the forum.
To answer your question, chances are that the probes (contacts) inside the tank are "fouled" and not reading the correct liquid level in the tank. However, if you've never emptied the tank, why take a chance?
Find a dump station. Usually at any improved campground, state park and many rest areas on the interstate. Connect the tank to the dump station and pull the valve handle. Then you'll know whether or not the tank is empty. Flush it well, and while you're at it, dump the remaining tanks as well.
That way you'll be "starting fresh, with empty tanks"....
Don't rely on the sensor probes to give you accurate tank levels. Eventually you'll learn, by sound, the level in the black tank and you'll learn how many days, showers, etc you can expect before your gray tanks need to be emptied.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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