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.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car