Search posts by bruceisla. He did the AC upgrade back in 2018 or so. He noticed the 12v problem.
You only need 3 sensors. However, you will need a new controller for the furnace which is located in zone 3 area. It is a silver box mounted under a sink in my coach.
I am not a fan of running cables through the ducts. I chose to run them on the roof and then through the roof into a cabinet. And then through the cabinets to get to a suitable location near the original location.
MBBeachbum's post on 5/7 brings up an interesting point. His coach was wired with eplex and Dometic sensors.
My coach appeared to be wired for both at the AC unit. I had 2 cables at the AC. One was eplex and one was something else. I was very confused. I never could find the other end of the Dometic sensor, so I assumed it was in the wall and out of reach.
You might look at your ac unit to verify you have 2 connectors, one in use and the other just there. If so, then try to find the end of the unused sensor cable.
2006 Newmar Essex 4502
2015 Jeep Grand Cherokee