Roman,
A missing expansion tank could definitely lead to gas fumes in the trunk and the cabin. There are two different types of expansion tanks that came with our cars. My 1969 has the older type that leads fumes to exit below the rear bumper. The newer type breathes fumes back into the engine. You should determine which type was originally on your car and install a replacement.
Regards,
Ted
1969 280SL w/ no fumes