Ours stopped registering anything at all, appears to be disconnected. The black water sensor shows either 0 or FULL, actual level means nothing, there is no in between. Gray water tank shows nearly full at about 12 gallons. I've recalibrated ours twice and pretty much have given up.
If yours is under warranty, you should take it back to the dealership. If out of warranty you should try the recalibration steps at your next full hookup site.
Rich & Yvonne
2006 Safari SE -Dora-
2004 4Runner SE