Go to this website and read what is their. It is for a coleman. But you can look at your wiring diagram and use it for yours.
http://www.modmyrv.com/2010/07/21/rv-coleman-mach-iii-thermostat-modification
I did it to my coleman that was the main AC. I just used the digital thermostat for the AC. I already had put a separate thermostat for the furnace. I left the manual thermostat still in the line. I just turned it to as cold as it would go so that the wire would be connected just like if the manual thermostat was not their. If I ever want to use the old manual thermostat. All I have to do is put the digital thermostat as low as it will go. That will keep the wire connected just like if the digital thermostat was not their. Then I can control the unit with the old manual thermostat. So I can use which ever one I want.