Another possibility is the black rubber grommet around the gas fill pipe and the hose that is connected to the bottom drain hole in the gas fill Area. That hose runs throught the trunk, behind the right rear wheel well and then to the ground. That hose could be disconnected or leaking.
I know, my grommet lost its elasticity and gas leaked into the trunk.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)