Without pictures of the spaces mentioned it's difficult to offer good advice but here goes: If you have one battery on the tongue, there should be enough space to mount more than one. The narrow boxes like Pondputz built accommodate this.
Installing batteries in a hatch compartment is certainly doable and I wouldn't worry about fire as long as all the connections are secure. Ask yourself if you have ever seen the results of a battery fire or have you ever read about one.
Years ago, I had an old Chrysler wagon that had a loose terminal connection (worn out battery post). I decided a screw turned between the (+) post and the connector was a good idea. What I didn't calculate was the height of the screw. When I finished my professional installation and closed the hood, the hood rested on the screw. Later, I saw a black spot on the hood and opened it to find that the battery had shorted and blown the top of the case completely off. If there was ever a chance for a battery fire that was one.
I would be concerned with is hydrogen gas ventilation and a couple of well placed vents should take care of that. Good luck with your batteries.
