What do you have showing on the SeeLevel panel directly. Not sure where yours in mounted. On my Classic 33 it is under the sink. If the black, grey, fresh have a reading try shutting down the CZONE and restarting. If the See Level has oPn showing then the connection between the SeeLevel panel and the sensors on the tank is disconnected, the sensors have failed or detached from the tanks.
Remove the SeeLevel panel from the wall and unplug/replug the connector.
What do you see on the Airstream Connect application?
2020 Classic 33 Twin, 2019 Ram 3500 Longhorn, ProPride