My batteries are installed in my Fifth Wheel Trailer just behind this front storage door to the right in an slightly recessed area. The flooring under the battery box area has open slits to allow fluids to drain to the ground under the trailer if necessary. This area is open to the whole storage area with 30LB propane tanks in a small sealed off compartments on each side of the trailer.
Having a vented battery box for each battery would be a must have for me. If this area was closed off to the batteries only then it might be a different story.
I also have an INVERTER mounted on the inside wall just a few feet away from the battery box.
If it were me I would locate a GC2 battery box that has a vent hose and houses two each GC2 6VDC batteries in the one sealed box. The vent for my battery box is to the right of the front storage bay door shown here. You can see a vent for the propane tank just above the left of the Propane Tank outside access door.

Everyone probably has different circumstances
Roy Ken