You are correct about that. I'm just going off my reading but if I do eventually set up 2 140's, I'll need 12's for sure.
Those bays are the width of the camper on the side lower rear corners. I thinki most TC's have them... The bays measure just under 7 foot on center but I was also considering two battery boxes end to end in a wood cabinet at the cab end of the camper on the floor, inside the camper. Any reason batts should not be inside a living space if they are in a factory batt box and again inside 3/4" wood ply cabinetry? I guess they would need to be sealed type..