I had a similar problem about 10 years ago, though not with my MB. I
have an '83 Firebird, and the guage would show full when I topped it,
thendrop to empty very quickly. After using the trip odometer as a
gauge until the weather warmed up, I drove it until it was as low on
fuel as I dared guess, dropped the tank out and removed the sending
unit. It turned out that the coil of wire that sent the signal from the
float to the gauge had physically slipped off its' mount partially. It
would make contact when the tank was full, but when the arm of the
float went down just a little, it was no longer making contact. A good
cleaning and some gasoline resistant glue and I was back in business.
YMMV, but this worked for me. Admittedly, dropping the tank on this
car was a tin plated pain in the.......You have to drop the rear axle
first. I hope I don't ever have to do that again.