I cut a board to match the contour of the bed so the battery sits flat, put it in a battery box with a cable to plug into the side of the camper where I mounted a plug and sat it in the right spot be for loading the camper. No hold down, where is it going to go. If I flip the truck I have more problems than a little battery acid. I have baskets that fit in the wheel well also and that doesn't allow it to slide much.
I used a 4 pin connector and used 2 connectors for each side of the battery and 12g wire, largest the connector would take. Since the connector was keyed it shouldn't be plugged in rotated 180 degrees but I discovered it could be if you didn't pay attention. I used the diagonal connectors so no matter which way it was plugged in nothing would be shorted or reversed.