[Mexray]

Venting is not a real problem when the battery box (s) are mounted outside, behind the propane bottles... assuming there are vent holes or passages in the box, cover, etc...

Search the Net for a box to hold the two batteries you want to use...

Of real concern, however, is to secure the batteries inside the box so that they don't move around when you're on the road - any dirt or small stones that find there way inside the battery box can act like sandpaper and wear away at the battery cases, most notably the corners, and could wear a hole, causing the electrolyte to leak...I've seen it happen several times...

Those plastic battery boxes have some relieved areas in the bottom to collect the dirt that finds it's way inside - however, the batteries still move around inside those boxes...

If the battery box you select doesn't have secure battery hold downs inside the box, you could spray some of that foam in a can, around the battery to keep it from moving - a solution I've used successfully - it's easy to peal away at replacement time...

I have two battery compartments in the front of our AS, but I added a third battery, installed in an alum battery box, behind the propane tanks...I got mine from Quick Cable Corp...and it HAS and internal hold down to keep the battery from moving around!