How to tow travel trailer with 2 adults, 3 kids, and 2 dogs

My friends dad had a dually crew cab. But they removed the rear window, and installed this extension back there. We would drive to the river and the dogs would be in that cubby spot. I would guess it went back a good 3 feet. Still left plenty of room in the bed he was a general contractor and hauled material constantly. Can slide long things under the cubby spot.