RV furnace fan comes on and blows cold air thru duct work because the same motor is blowing air thru combustion chamber to purge it before gas flows & is ignited.
Then after thermostat set point is reached the gas flow is secured but the fan motor continues to run (timed event) to cool down heat exchanger....that results in warm/cool air being blown thru duct work.
If this is what you are experiencing and think it is due to 'outside' air ......it is a normal RV furnace cycle.
If this is NOT what you are experiencing then explain what is going on/how you are getting outside air in AND not run furnace anymore until repaired because outside air getting inside RV via furnace means combustible products (propane fumes, CO) are coming in also.
