I haven't read a 2015 manual.
We have a 13 Edge. A lot of people were putting the foot on the brake, as natural when they put key into the acceory or off position with trans in neutral and guess what the radio and lights and everything else, including odometer was working. It drained a lot of batteries. We did it one time.
So maybe they said to disconnect the batter for that reason.
We have a charge wire from Mh as, most or all others do to the battery. It powers tail lights blinkers brake lights etc. Supposedly the headlights won't work in the accessory position but we cut them of anyway and put the ac outside vent door, closing it, into recirculation mode before cutting off the engine into accessory mode.
You might call Ford Customer Care and inquire, Most Dealers guess or make stabs at it unless they sell several to RVers.
there have been others with 15s on the forum and hadn't mentioned disconnecting the battery.