Welcome to the forum.
The tank indicators are notoriously inaccurate, especially the black tanks. TP and other stuff gets hung on them. The only level indication I look at is Full. I'm not familiar with your model but it seems a bit odd the kitchen sink dumps into the black tank. Trying to figure out why things are done the way they are will drive you bonkers.
Keith and Lisa Ward
2019 Pinnacle 36FBTS
2021 Ford F350 XLT CC LB DRW 6.7L Diesel
Demco 18K Autoslide Hitch and Timbrens
2019 Ford F350 SB 6.7L Diesel (Gone)