Could it be that the smell is coming in through the vent up front. With mine,the fresh air supply came from the front inside of the grill. It's possible the fumes could be coming from there. I wound up rerouting the intake from the wheel well. Albeit it was for a different reason.
1990 28' Georgie Boy, 454, 4bbl, Thorley Headers, Recurved Distributor, 8.75 mpg. RVM 76