My batteries are on-board/inside. The main concern is should the engine's alternator regulator (Alternator produces 14.40 amps) fail to shut down after a full charge the batteries will produce hydrogen gas. I'll be running a 3" PVC pipe, with internal 12 volt fan controlled by a toggle on/off switch. The PVC will run to an outside wall, capped with vent screen on the outside. I'll click it on while driving and will probably shut it down (Depending on noise factor) when plugged into shore power, as only a 13.40 Amp charge is being sent from the converter. I think it is good to check for hot batteries by hand to make sure they are not over-charging or have lost fluid. When I win the lotto I'll install all sealed batteries.
I checked out a couple of battery vent kits you mentioned and they appear to be just what the doctor ordered.
"If everything seems to be going well, look around, you've obviously overlooked something."