It looks like Mercedes uses a float arm and variable resistor system to gauge fuel level. This element is located inside the fuel tank.
The instrument cluster taken this signal and processes it to determine a fuel level. That process includes a function analysis tool to dampen out fluctuations. You wouldn't want the tank to read empty when going up big hill and empty when going down a big hill.
My guess is the float resistor isn't functioning correctly. Usually it's a surface type resistor with 'wiper' contacts mounted to the float arm.
- possible erratic contact between the wipers and the surface of the resistor element
- possible bent float arm at the pivot
- possible 'water logged' float
- possible problem in the wiring between the top of the fuel tank and the instrument cluster.
Not totally sure about your car but generally the top of the fuel tank is accessible through a panel under the rear seat (on a sedan). I do know lately automakers are deleting the access panels to the point that you have to drop the tank from beneath the car for access.