It's a known problem on the w211 chassis. Do some googling and you will find plenty of discussion - mostly on other MB forums. The smell originates from the fuel pumps/fuel sender/fuel hoses. Fixes vary from replacing some hose, to replacing the two pump/sender units, to replacing the fuel tank.

Remove the rear seat bottom and you will see two access openings to the two pump/sender units. Remove the covers and you can inspect and likely see what is going on to diagnose.

I have this same problem on my E55 wagon right now.