I believe, based on my fibeglassing experience and ,having completed that step, that wetting out the overlap area and eliminating or bubbles from the butt block fillets and wetting out the transom will be much easier with only 12″ overlap. And that long 10″ strip of extra fiberglass will come in handy for something. I ran a strip across the inside of the roof in front of the hatch.
I overlapped 12″, but I actually believe (just personal opinion) that the camper bottom is so strong that you are really only glassing it for road debris ding protection and strength at its joint with the shell. I’d be willing to bet that no overlap or maybe just a 1″ overlap on one camper bottom side panel or the other would be just as strong. If I do a second camper that’s what I’d do.