on our Palazzo, and similar to most any 'dual' thermostat RVs, only the 'top' thermostat operates the FURNACE, even though both thermostats are identical, unless on some longer/larger units, which actually DO have two furnaces - and some do.
The object of the thermostat setting is to limit it to 'AUTO' only on the Fan Mode. If you accidentally leave the Fan Mode in either 'Low' or 'High', you are going to have the AIR CONDITIONER fan running - not what you want. Leave the Fan Mode always in 'AUto' when you are not using the Air Conditioners, and only want to use the Furnace. The furnace has it's OWN fan controller, you have nothing to do with controlling that - it cycles on BEFORE the firing of the furnace, and stays on well AFTER the furnace fire goes off - all completely normal and REQUIRED. You don't want hot air sitting in the vent system, you want it pushed all out, including any gasses after the furnace is no longer firing.
two Campers, two Electric cars
former diesel pusher traveler