I would still contend that it is the gas cap.

I can't think of anything else that would backwash out of the tank except for the nominal fuel pressure that is maintained while in operation.

I don't know if the seals are sold seperately, but OEM gas caps aren't very expensive.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car