The ML fuel sender/fuel pump assembly is accesable through an access hole under the left side of the rear seat. The seat is removed and the carpet can be pulled up, and there is a metal cover on the floor which is sealed with body sealer. I use a hot air gun to soften the sealer, then pry it up with a gasket scraper.
The top ring which holds down the sender/pump unit is then removed with a special MB tool, it would be difficult to remove the ring without this tool on the ML especially.
There is a defuel procedure available for the C Class, I would recommend, if you need to remove a sender, in doing it correctly.
There is a defuel hose which attaches to the right sender unit, then a special fuel cap with a hose attached. Compressed air is introduced into the fuel cap with hose, and this will allow fuel to begin flowing out of the defuel hose and into a fuel caddy or a large fuel can.