Ray’s advice to look for a leak in the hoses and fuel tank is good, but the smell isn’t necessarily coming in through the windows. When the windows are closed, the heater/air conditioning/ventilating system may be pressurizing the interior and keeping the smell from coming in through another opening. I had a decrepit old van once where I could keep exhaust and blowby smells from coming in either by running the ventilator fan, or be turning the vent windows around while driving to pressurize the interior.