I have an 01 ML430 and my gage does the same thing. When you fill up it goes to full then slowly falls off. Some times after I fill up it goes to full and when i turn off the key and restart it goes to empty. A lot of times i'll pull the fuse for the dash and it'll register the right amount. But as soom as you fill up the circus starts again. I guess the only fix would be a new fuel sender. But don't quote me on that one.