Thanks for the response.
Actually, these will be something along the lines of the Battle Born 100AH 12V LiFePO4 lithiums. Going to this much effort to fab/build a new battery box, may as well get the most bang for the buck.
(4) of the BB LI's (124#) are only 20% more weight than than the (2) lead-acid Group 27's (100#). So while ventilation will not be as critical as lead-acid or AGM, I am still thinking of some means for ventilation - maybe holes in the battery box sides?
Coming from a cruising sailboat when we were dragging around a pair of 8D's (each at 150# and the size of a microwave oven!) battery tech has come a long way.
That said... as I think about it, I have not seen any ventilation in any of the photos of batteries-in-the-steps in Class B/C RVs. Just seems like batteries jammed in tight with a hinged lid.
Heck - maybe they count on the hinged lid acting as a pressure relief vent...