Bigger Truck

You state that you are looking at used vehicles. Definitely need to check the door sticker to ensure you have the payload package needed for a camper setup. Since you're going gasser, 2011 and up Fords come with an excellent 6.2L v-8 motor. Until recently, they have been the leaders in payload as well. 2013+ HD Rams have an optional 6.4L Hemi that you should really look for. Starting in 2013 for 3500's, RAM has had an upgraded Frame. The upgraded frame was passed on to 2014+ 2500's as well. This frame has bulked up their payload to insane levels which currently puts them and Ford on pretty even footing rating wise. However, RAM does indeed have a stiffer chassis, so if you go off the beaten path with a camper, it would certainly be a consideration. As for GM, They still have the tried and true 6.0L motor, however the chassis seems to typically have a lower payload than the other two for SRW vehicles, at least for 2013 and up. I think this is more due to the smaller tires they tend to spec versus RAM and Ford. But the placard on the door is the final word on payload as far as the OEM is concerned.