2017 New truck says 5th wheel it not connect? Update 2-14-18

Check truck plug w/ 12v lite tester 1st. Verify all the pins work. Then move onto trailer. It's been my case when either the left or right turn signals flash fast that a bulb is burnt out on truck. Upon edit: you may have a trailer brake wiring issue. Had this occur to me a year ago w/ snowmobile trailer. Kept getting trailer connected/disconnected msg on dash. Ended up that only 1 brake out of 4 was working.