had the same issue with 01 ML430 - 96k miles, fuel gage did not move after filling the tank. Found a reset procedure on MBForums, very simple to do. Locate the connector to the fuel pump. Left rear wheel near the front of the wheel housing. Un plug it, turn the key to position 2 then re-connect it. Gage should reset.
I have the same problem. After 50 miles it starts working again. someone suggested the gas attandant doesn't turn the cap tight enough and it causes a vacuum. Need to hear 3 clicks when tightening the cap.