Black water - Grey Water ??

You are finding out that those tank gauges are notoriously inaccurate. The water tank level is often correct, the black and gray water tanks not so much. You can try cleaning the tank(s) and that may clear the crud hanging on the sensor rod and make it work better for a while. A cup of water softener (powder) and a cup or two of Dawn is often used. Add 5 or 10 gal of water and let it slosh around when you drive.

There are more accurate meter systems, search in the DIY forum and you'll probably find mention of them.

BK