Thermostat has no 12v DC

[Jbaker77]

I have replaced the entire A/C system: roof unit, control box and thermostat on a '01 Mallard 27H after a capacitor melted on the roof unit.Now Now I have no 12v power at the thermostat. Where does this power come from???

[dougrainer]

It depends on how the OEM wired it.
1. The OEM wired the 12 volt to the wall tstat from either the furnace or Main 12 volt fuse panel and designated one of the wires to the roof with 12 volts
2. The OEM used the Furnace 12 volt supply to the Tstat OR upper control board depending on if a Coleman(RVP) or Dometic roof AC
3. The OEM ran a dedicated 12 volt wire from the main 12 volt fuse panel to the Upper control board of the AC unit
In your case, I would check the furnace wiring and furnace 12 volt fuse at the Main 12 volt fuse panel. Did you replace your system with the same BRAND AC system?
Last, you just joined. ALWAYS post Brand and model of Appliance and Brand Model and Year of RV. ALL RV's have different specs and wiring and appliances. Doug