If your voltages are in the 14.2 VDC range that means your converter is operating and is charging your battery. As the battery becomes fully charged, this voltage will slowly drop to around 13.5 VDC, until you put a load on it. SO, make sure your battery does take a charge after a while.
By the way, if you are measuring from the GROUND WIRE of the thermostat to the RV GROUND and get any voltage measurement at all then you have a ground problem, which can be dangerous if not fixed. I would make sure of my GROUND and COMMON connections and then make the voltage measurements to figure out what's going on. From what you have told me, you have a ground connection problem.