If you are worried about Hydrogen, I would just replace the two acid/lead batteries with AGM batteries which are sealed.
The battery compartment is well ventilated to the outside and should not be a problem. The box was made of galvanized steel and can rust with age (23 years old in you case). If your converter/charger is overcharging the batteries you would be able to smell a rotten egg odor coming from the batteries and they would be warmer than normal if overcharging. Also; you should see a sulfate power forming around the battery-posts connected to the charger,
Our trailer was stored in a lot without access to shore-power so I added a manual battery-disconnect. A screw type that would clear the door opening when pulling out the tray. We took the batteries home in the winter.
If you leave the trailer plugged in and charging all the time when stored that can create the problem unless you upgraded to a decent 3-stage charger.