Several companies make the required adapters or you can go to an RV salvage place and pickup and old electric brake magnet and wire it into the brake control line. GM's are looking for the low resistance of the magnets before they will pass any current to operate the brakes. This has been fixed in the 17/18s.
2018 Chevy 3500HD High Country Crew Cab DRW, D/A, 2016 Redwood 39MB, Dual AC, Fireplace, Sleep #Bed, Auto Sat Dish, Stack Washer/Dryer, Auto Level Sys, Disk Brakes, Onan Gen, 17.5" "H" tires, MORryde Pin & IS, Comfort Ride, Dual Awnings, Full Body Paint