When these units say they are "prewired" for the second AC, I don't know if that means they are only pre wired for power, or if the wiring for a second thermostat is actually run already within the wall as well. Maybe someone else can pipe in about that part?
I found a listing for a 2018 HT 5th wheel that has the 2nd AC installed from the factory and one of the pictures show the 2nd thermostat mounted directly above the light switch in the bedroom. That is the exact location the thermostat for our 2nd AC in our 2017 Eagle TT is, so I assume that might be a normal/common location for the 2nd thermostat?
Not sure if it would help, but I would pull the shroud for the bedroom ceiling vent and locate any wiring that is already there. If you locate what appears to be thermostat wiring running in the direction of the light switch, then maybe pull the light switch to see if you can find the other ends in that area? I know that hole isn't very big and might not help, but it's worth a try?
https://www.rvtrader.com/dealers/Mil...STS-5000053135