Vintage Cars: 83 380SL Fuel Odor, vacuum test, vent lines

Question
I am the 2nd owner of an 83 380SL with 80K miles. I've owned it since 1989 and it's serviced by a MB specialist. He is stumped by a consistent gasoline odor in the trunk and over the past year or so, have invested much time and effort in taking care of the problem. It's been tested with vacuum and pressure, smoke tested, monitored, etc. Probably all the gaskets, hoses, lines have been tested or replaced.  Still, I have the odor in the trunk (when we started this search, the odor was in the interior as well) that is noticeable when the trunk is opened and monitoring levels indicate 15 ppm.  Any ideas?

Answer
Larry
well this is a common complaint, but yours sounds more significant.

areas to check - every clamp on the vacuum/vent lines from and to the tank, vacuum test, dont assume they are good just because they look good. A lot of times its the connections. Faulty gas cap seal is another area. Fuel gauge sender gasket - another area.
Leaking or faulty filler neck into tank
Fuel lines to and from fuel pump as well as fuel pump.
Gas tank exhaust valve - sometimes gets clogged with dirt. I am pretty sure you car has one, it was the problem on my 73.
Although rare the tank itself may be at fault, upper seam issues and or pinholes (this would be above full gas tank line)
Hope this helps
Good Luck
Ron