GM has codes, called RPO codes, which means "Regular Production Options"
They are 3 digit codes of letters and numbers, on the sticker in the glove box.
The RPO code for the camper wiring provision is UY1
Check your glove box sticker to see if it has that code. The codes are listed alphabetically.
It would be odd that a dually wouldn't have that - no idea why the dealer that had my truck ordered it in without that option. Duallys are primarily used for hauling campers or towing a 5th wheel, so a normal person would think that the guy ordering the trucks should select that checkbox when ordering in a DRW!