Going to high-strength steel or steel alloy will reduce weight. I'm sure there is more aluminum in the W220 than the W140, but aluminum didn't account for a 500 pound weight saving. Again, there are no aluminum body panels.
The simple way to find out rather than debating it for the next six months is to take a kitchen magnet (like the one that holds up your kids' drawings on the frig) and stick it to the body panels. When it sticks, it's because they are steel. Whatever it doesn't stick to is not steel; it's glass, or rubber, or aluminum.
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