If your tank is not full, you could open the inspection hatch in the tank en see if your level sensor is not stuck in the topside. Sometimes a little piece of dirt in the tank makes it for the level sensor to be stuck in top position.
On our previous Bavaria, we had a vdo levelsensor and on this one the float and the levelpotentio meter got detached, also displaying the same level at all times..
From my experience, usually the problem is with the level gauge.
Good luck!