First, I cut the 14 x 14 hole carefully avoiding structural members.
On my bus, there is a stringer that runs from front to back right down the center so the hole is off center to the right side to clear the stringer and between the ribs that run side to side.
I had the inside ceiling loose at the edges to push the 1 1/2 sheet foam into the void between the frame members.
While I had it down, I ran the thermostat wire, the 12VDC wire and the 120VAC between the roof and ceiling, down the wall, under the floor and then to the breaker box location for the 12V and 120V.
The thermostat control wire goes over to the thermostat mounted on the right hand wall between the two seats on that side.
It is about 18 inches down the wall and controls the on/off, fan speed and temperature.
Then I framed between the roof sheetmetal and the ceiling sheet metal with walmonized 2 x 4s ripped to 1 5/8 inches as that was the distance between the two pieces of metal.
I drilled a hole in the one on the right side as that is the side that the thermostat and power wires are on the AC unit and left them dangling down about 2' to make sure I had enough slack.
Then, I placed the AC unit on the roof over the hole and got inside and centered it over the hole.
After it was bolted down, I connected the wires, then put on the trim on the inside.
You do have to have all 3 wires because the 12V is what the control board runs on that controls the AC.
I bought 6 conductor wire for the run from the AC to the thermostat, but only used 4 of them and have a couple of extras in case one gets cut or something.
Hope this helps!