Having the fifth wheel only has the recessed area just inside the door under the fifth wheel hitch for installing wet cell batteries...
Most are already vented and has an opening for fluid drainage...
I thought this four 6VDC battery install was neat for the fifth wheel when I was looking years back using the TROJAN T-105 batteries....
More info may be found here at modmyrv.com link...
modmyrv link
Since I love battery disconnect switches I would do something like this for two groups of two 6VDC batteries in series...
Of course you will find many different ways to do this...
Roy Ken