Batteries are above the sealed area of the tanks and detectors, so any venting from batteries would not get into the tank location.
When i said leave the covers off, i was refering to the white round hatches in the floor of the consol. Any petrol fumes will rise and vent out of the consol.
If you want to add any vents, ensure it in the "square platform that the consol moulding then sits on. Any higher and it will not vent the tank area.
Cold, wet, cold wet ..... Hang on, no i'm not, I have a cabin and heat