I just purchased a 2014 GMC Sierra 4x4 crew cab with the Z71 option as well.
However, I did a little research prior to my purchase and found the attached service bulletin which addresses the dinghy tow ability of this new model truck. This bulletin was found on the GMC truck website under the customer service tab. I also had my dealer service department do some research for me as well and they generated a option build sheet for my truck and a newer version of the service bulletin for me.
You are correct the battery must be disconnected and the keys must remain in the ignition accessory mode before you can flat tow this vehicle.
It was much easier process with my 2007 GMC Canyon 4x4 Z71 truck just put in neutral and go!2014 GMC Dinghy Towing Service Bulletin.pdf
