Leisure battery compartment

I've found some storage space on the side of our van where I could fit 2-3 decent sized leisure batteries, but its under a seat unit that isn't sealed off from the accommodation unit... I've read the recommendations about battery storage and just wondering how others do it? Would this storage compartment really need to be sealed off and vented?

This compartment is accessible from inside AND outside the van (door on the outside).

I noticed a couple of heavy 16sq cables in there so someone may have had batteries in there in the past. There is currently no backfed power on the wires so I'm guessing they're not hooked up to the 12v leisure battery that's connected up to the camper and is in a locker on the other side of the camper... I don't believe my folks ever had batteries in there as it was always used to store the telly... The door was never used and actually was secured with screws. I un did this and got the lock working so now it's accessible from outside once again.

Its above the gas tank access panel if that makes a difference?

Ours is in a compartment with a loose plastic cover in the floor under the bed and not sealed, there is a plastic pipe coming off the Battery and going down by the side of it.

One under passenger seat and one under the back seats. No vents and professionally installed!

Ours are in a wooden compartment under the bed with a small vent hose from batterys to underside of van . I would think you can put yours where you want to no problem maybe with a small vent hose