Most likely, you have one of these above your panel that's under the existing A/C unit. The special 'phone' wire that goes from the thermostat plugs into the box circuit board for the existing A/C. from that box to the new A/C, there should be another 'phone' cable, and that should plug into the same box which should be installed when you install the new A/C so you can control it from the same thermostat. The box is at this link:
https://www.amazon.com/Dometic-33120.../dp/B00HSO05R4
To make this work, you will also need a remote temperature sensor, which may also be installed already.
https://pantherrvproducts.com/dometi...rature-sensor/ Ours, which was factory installed in our rig along with the factory installed A/C, is mounted in the ceiling of the bedroom, and it connects to the box to be installed that is in the link above. In the box, there will be a dip switch which identifies the new A/C for the existing thermostat, so you can control the A/C. This manual will explain the installation instructions:
https://www.dometic.com/assets/12/75/3315085_71275.pdf
Good luck.