That helps, 2 plans.
1 get a couple of pieces of angle iron and put the batteries right behind the propane tanks. Use the angle iron for to box them in with and use plastic battery boxes. You may be able to move your propane tanks forward a little too to give you some room.
Second is to use a couple of vented boxes inside the storage area in the front. They sell a box that is vented with a hose. You would need to drill a hole in the side and vent it out but it isn't too hard.
If you are going to all this trouble and need more capacity 2 6 volt batteries are the way to go. Much more capacity than 2 series 24's and the size is not too much larger than 24's. THey are taller though so make sure and do your measurements
Chris