I am having the opposite problem on my 1998 C230. The gauge won't read below a half tank.
I talked to the dealer on Monday while picking up the E300, and they seemed to know immediately what the problem was.
Problem? 5K out of warranty and a $590 bill to repair the two sending units. And yes, that's the going rate in these parts, not even the indies will do it for a dollar less.
On the "good" side, MB is considering paying a portion of the bill as this is a common problem, though they don't admit that. Their reason for chipping in is the low "over warranty" mileage.