[C&P RV]

If your rig only has a single toilet, you only have one black tank. Were both blacks showing full before you dumped? If not, I'm guessing you have the same issue most people have, especially if you haven't rinsed the tank, which is the sensors have toilet paper or debris on them, so not reading correctly.
For the gray tank that occurs less often.
Look under your RV for the location of the black pipes coming out of the underbelly. Somewhere near one of those lines is a gray T-handle that is the valve for the second gray. Usually it's near the kitchen area, but maybe the bathroom depending on your floorplan and the location of the sewer hose connection.