Help on 560SEL

[JDDrewer]

I spent all day trying to locate what I thought had to be a gas leak, due to the horrible gas mileage and the smell of fuel when I first started it. Nothing. As I crawled underneath it and shut it off in my garage, I noticed a large amount of water had puddled directly underneath the end of my tailpipes. I move the car and let it run for awhile. Same puddle, different water. I know I'm not gonna like what I hear, but any thoughts as to what is causing this and my horrible mileage?
Thanks for your input. Anything helps!!
JD