I would look under one of the bench seats. If you go with AGM type batteries just a simple vent for fresh air. If you stay with wet batteries then you would have to seal off the whole area and properly vent to the outside for battery fumes. A drain on the bottom to the ground somehow as well would be nice.
The joys of the truck camper. Not much extra room for anything it appears. With all the stuff I carry on my trips I would most definitely have to drag a utility trailer behind the truck to support my habits.
Roy Ken