The key should be in whatever position is needed to unlock the steering wheek but still have as much of the car's electric system as possible turned off. Also set the dash vent system off/closed so that dust and exhaust behind the motorhome does not get sucked into the car. If there is a separate control for fresh air vs recirculate, set it to re-circulate.
If you do not already have a tow brake, I storngly recommend one for my safety as well as yours. Anything that helps stop the rig sooner is a GOOD idea!
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL