I'm not sure of your setup from what you describe but there should be an inverter power panel located under the forward left dinette storage the same as my 1991 17 foot Bigfoot
and yes I have room for a second battery as well.
As for H2 gas I spent my working life manufacturing H2 gas and can speak to all the fear that folks have. Yes battery manufactures must warn of H2 liability is everything.
Now for the facts.
I would not be concerned in the least storing a battery in that storage space.
H2 gas is lighter than air and dissipates quickly one would need to spark the vent directly or hold a match to it for it to ignite.
A 12 volt battery or 6 volt does not produce near enough H2 under normal conditions to be a problem. If the vent were plugged that would / could be an issue.
Most folks assume the moment there is a serious accident with batteries that it must be the H2 factor I would need to see facts to support this.
In reality most accidents involving batteries are the result of power shorts. I've seen a positive battery cable in a car glow hot due to a short connection, this is a much bigger issue with batteries. Even when jumping one battery from another always make sure the ground is good. When I boost a battery I always use the car body for ground never the battery terminal.
I've also seen 1200 volt breakers explode due to shorts circuits these were industrial alternating current no H2 issues. One should never assume it was H2 that caused the problem the moment a battery explodes in most cases it was a short not H2.