Check your gas cap. My 1986 300e has a cap that if you insert it and turn it the wrong way, the center stem is dislocated (can't think of a good word to describe it) and the fumes will get into the cabin.
You have to re-center this sort of cross piece.
Just in case, open the hood and sniff around. If the smell is ok, not overpowering, then the fumes could be filtering in from the back.