also it looks like your gas cap may not be vented. put it on a tube that cover the rubber and blow in and suck out and see if it goes both ways If its not vented then the paper was releasing the pressure enough but the rubber is not.
Most vented ones have a hole in the center to vent. I don't see that with yours.
If its not vented then the pressure is building up enough in the tank to find just a spot to get out inside the cap. Normally windows up and it venting by the cap you should not have a smell inside the cab
