The way I check the controls - the on/off and and auto/propane are manual buttons.
Once I press on and then auto, when panel comes to life and 120V is automatic default.
To turn 12V heating, there is 3rd manual button, but that will stay off all the time.
That part sounds easy at this point.
Now have to figure out how to run the 120V cable.
Can't be bed-mounted becouse my camper fits with minimal clearance and can shear the cable on bed edge.
So drilling small hole in back of the cabin to run cable with weather-proof female connector, than another hole on the bottom of fridge compartment and male connector under fridge floor plugs into female from the cab.
This way it is easy to plug and in case I forget to unplug (when dropping the camper), they will simply pull off.
It cooled down to 110F, so if colder weather stays, I can start doing camper side.
