There was a long thread about this a month or two ago but I can't find it. Maybe someone with more time can bring it up. It seems (seams, ha ha) to be quite common. My 2012 Laramie had a slight seperation in the bed seams same as you discribe. I had RhinoLiner sprayed in the bed and Ziebart under and can no longer see daylight, its now sealed up. Not saying it's right on a new truck though.:4-dontknow:
Jim in Pa
Jim in Pa