The corrective measures talked about here to "level out" the 2015 GM pick-up bed crown sound worrying to me. Think about it: your 3000, 4000+ LB camper will be sitting acutely (severely) on just 3 support points in the sheet metal bed (even using high or low-density XPS foam).
Those with the cracked sheet metal: what is directly under those cracks (what if anything is under the cracked sheet metal, underneath your trucks at precisely the crack) ? Any photos of the under-bed structure under the cracks ?
Further to this bed "crown" issue, I cannot find anything published by GM on it (also, nothing mentioned in the GM Upfitter's manual).
I'm putting this on my "follow" list...