We have 6V AGM batteries in a series parallel arrangement with balancers on each string of 4.
We have no issue with vapor as the batteries are AGMs, The balancers are in the battery compartment with the batteries.
If you have acid vapors in the battery compartment I wouldn't put any electronics in there.
For your setup, I think from my experience the best setup could be :
Two balancers on each of 6 batteries in series parallel with the middle negative poles connected together.
See my drawing attached.