I don't know about having a "fancy trailer", but it's what Thunder Creek sells.

The main 12V battery is not mounted there. It's "taking up space" as you suggest up on the top shelf area of the compartment. It is a truly handy and accessible mounting point. I presume what you're looking at is the breakaway controller box, which does have a small battery to energize the electric brakes when the pin is pulled out of the socket in the case of a breakaway trailer. Re. combustibles.....the whole inside of the cabinet is a wiring menagerie. The 12V battery sits up there, there's lights & light switches, starting switches, electric motors (pump) and on & on. To my knowledge, when you have a fuel leak it usually runs downhill. That would take you to the c-frame area where the junction box is now, not in the upper regions of the cabinet. 

Not many here have commented on how the current location, which I pointed out to friendly TC folks, allows for DEF to leak down on top of the junction box. We are not sloppy and don't pump DEF all over the place, it just drips somehow....right down to the junction box as you can see in the photos. The white stain you see is dried DEF. What do you think the outcome of that will be?

That junction box is about the same length and width as a cell phone. Maybe 1.5" in depth. It won't take a lot of room. When there is time this winter for such projects, I'll report back with how the "move" went. Ironically, both the in & out cables from that junction box originate & end above the frame.