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I have a 2016 Silverado, and on 2nd trip with my new 5er got a similar message. Stopped and checked, and the plug was not fully inserted and trapped by the molded-in tab (which gets trapped by receptacle "door"). I was impressed that the truck could detect the intermittent contact. So I would look for loose connections in the wiring of the in-bed receptacle.
Good luck!
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