Alan,
For starters, All boats and Many motorhomes carry some batteries "inside".
If you want to spend the money, AGMs do not outgas and can be anywhere. In Europe, there are several models that carry the vehicle battery under the passenger's seat.
If you don't want to spring the money for those, just build a "pretty much" sealed box inside and vent it to outside, this does not take much to do and I have built a few. The only time a lead/acid battery outgasses is during charging and that is hydrogen which will leave fast if you give it any chance at all.
One of my favorite stories is about a person that had batteries stolen. He collected some shot ones and used them as replacements for each stolen until they stopped getting stolen. Then he never has a problem again.
Matt