After disassembling the battery box, I cleaned the batteries and placed them on the floor at the foot of the bed. Lengths of 1" x 2" were then screwed to the floor and bed box to keep the batteries from moving.
I also took this opportunity to reverse the positive and negative of the battery bank, and also to use different batteries as the "terminals" of the series/parallel string. Put another way, I shuffled the batteries.
The box is vented to the rear of the bus with a 12-volt muffin fan from Shaque du'Radio. A screened, 3" hole on the right side of the box allows fresh air entry. The hole where the wires enter/exit the box is sealed with spray-in foam.
Right now the battery vent fan is manually controlled with a toggle switch. Next week I'll add a voltage-sensing switch to turn on the fan only when the batteries are charging and generating oxygen & hydrogen.
I added gasket material to the sides of the box to ensure a nearly air-tight seal between the box and bus door.