I'm going through the same thing right now with my 560SL. My wife has been smelling gas for two or three weeks now, but I don't seem to smell it (but I don't have a garage). Well, last week I noticed it wasn't starting up right away, 2 or 3 seconds of cranking instead of immediate. This morning, I had to crank it 3 or 4 times before it started. This afternoon, it took forever to start it, then it was fuel-starved going up steep hills.
Tomorrow, I'm going to replace the fuel filter that I got about a month ago, but hadn't put on yet. I'm hoping that will do it, but I'm not holding my breath. A month ago or so, my fuel pump was whining, but I haven't heard that for a couple of weeks now.
I'll be following this thread closely.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino