Using the Nissan website, when you build your own - there is no "tow package." Just the option to add the hitch and wiring as accessories (for $340). I don't think it's like outfitting a Ford Super Duty, where the tow package adds other things such as transmission coolers, etc.
You are just as well to buy your own hitch and install the wiring than to have it pre-configured on the truck.