The two seperate thermostats are, for one thing, because air conditioning used to be an option not everyone bought when the trailer was ordered. A thermostat for only the heat could be used on all units, and the air conditioner thermostat could be added.
Also, the furnace runs on DC, with DC voltage going to the thermostat, and the air conditioner runs on AC, and has AC voltage running to the thermostat, with IIRC, 18 VAC. AC and DC don't play well together.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.