Have you found a puddle of gas under the car when it sits for awhile? Could be loose fittings for the hoses so they leak and smell like gas. I know my diesels like to do that so I just replace the hose clamps especially if they are the non screw down type.
Depending on the condition of your tank, fuel tanks can be repaired by thoroughly cleaning the inside and having the inside of the tank professionally sealed. As stated before, you may just have a loose hose clamp.
have you checked all of the fuel lines for weeping? i had a similar issue with my 66 230 sl. turned out the main fuel line leaving the tank was weeping where the hose clamp secured the line to the tank.
I'm not sure if the coupe and cabriolet have the same plastic vent tank in the trunk that the 113 has, but the hoses to the tank and the tank itself can weep. On the 113 the tank is in the right rear corner of the trunk.