Battery question in conversion camper

Yes that will work. You do want the batteries and their vent tightly partitioned off from the rest of the electronics, so the hydrogen doesn't contribute to corrosion of the electronics. I would recommend you get AGM batteries, to avoid the possibility of acid spillage in the trailer.

I would recommend the fan be on the "inside" wall of the compartment, and a vent hole on the outside wall. Have the fan keep a positive pressure on the battery compartment.