Randamy, the correct answer is "it depends."
Erring on the high side will virtually never be an issue and will provide the hardest, most stable ride, with your truck (small, relatively light capacity tires in front).
With a relatively heavy weight camper, I'd run max pressure or close to it in all tires.
Look at a load/inflation table for that size/load rating of tire and consider that your empty dually diesel will have around 5klbs on the front axle and 3500lbs on the rear. Your camper will add nearly 100% or it's weight to the rear and little to the front, if any.
One thing to note, the OEM Nexen tires are kind of known to be low quality tires. If you have any vibrations or weird tire wear, it's probably the junky tires and not your truck.