Let me tell you! I went nuts with a gas smell on my mother's 1996 E320 that would not go away! One day, it was so bad, it filled the entire garage with the foul odor. To my surprise, the answer was very simple. The rubber seal on the gas cap had a split in it and was leaking gas vapor. To my surprise, this never triggered a Check Engine Electronics. Oh well! So i replaced the cap and VOILA! No more gas odor!
And yes, the odor was coming from the right rear passenger area. The reason? As small amounts of gas pushed their way through the cap (from heat expansion) the fuel would drip down the syphen tube by the rear wheel well.
Just remember, because it is a Mercedes Benz, doesn't mean its any more complex than another brand. KISS!
=)