Jim,
Batteries do not love heat, it shortens their life. Wherever you put them, it must be well vented if lead acid. You may get by without much service, if the area that you choose has limited access, by going with AGM type that do not need to have water added or require venting. Those are the design constraints.
AGM Info...
AGM Batteries. A VRLA battery (valve-regulated lead–acid battery), more commonly known as a sealed battery or maintenance free battery, is a type of lead-acid rechargeable battery. Due to their construction, they do not require ventilation, can be mounted in any orientation, and do not require constant maintenance.
It may be possible to have a small bank of AGM under the bed, in the closet, or in the storage bay. You should try and keep the wire run to couple these to the existing battery bank relatively short if possible. Wire size diameter grows with distance. Hope this helps.
Later Ed