Kenny,
The 110 VAC wiring and I believe thermostat wiring are stubbed into the roof cavity near the vent where the AC would be installed.
Instructions for installing the AC will come with the AC. Dometic is the brand used by Heartland and many other manufacturers. Some owners have installed Carrier units in this location as well.
I believe the opening is framed out to 14"x14". To my knowledge, all roof mounted RV ACs require this opening. The opening is also reinforced for the weight of the AC with the bowed roof trusses being doubled up fore and aft of the opening, then boxed around the opening.
Your dealer or any service center can install the second AC in about an hour and many owners have installed their own.
Have fun and happy camping.
Jim