No fans for venting. Batteries are in a plastic box with a cover that has a outlet for a flexible 2" tube that goes thru the wall, tube attaches to a round louvered fitting.
Thats typical. My trunk mounted battery in my 65 Coronet is set up like this. I would assume camper batteries would give off more hydrogen during a given time period, as they are typically charged for a much longer time. Still I dont think a fan is necessary. WE could get into the question about poor/old/worn batteries excessively gassing during recharge, but I think at that point in time most would have already replaced them as they wouldnt perform well enough to use.
Additionally, there are sealed gel batteries on the market, that give off no gas, and can be charged whenever - not just when they are low on charge without hurting the batteries. They are about double the cost though.
Bull, are camper batteries typically mounted inside a sealed box, with a vent? Not just the typical plastic boat boxes?
The coach batteries in my Winebago Superchief are under the steps to get in the motorhome. I think the steps are pretty sealed and there is a tube leading out of that box. I am thinking about putting one or two under the back seat of the van. I wouldnt want any smell coming from them.