You just joined. Remember, for accurate help you need to always post Year/Brand/Model of RV AND the brand and model of the appliance.
IF the Furnace runs and heats for 1 to 2 hours, odds are it has reached temp and it goes into a cool down cycle. There is a Hi Limit switch to prevent the burner chamber from being TOO HOT after the furnace reaches temp. This cool down can last from 5 to 10 minutes and then the furnace fan motor shuts off. IF the HI limit fails to operate, the fan will stay ON until you switch it off at the tstat. This is the most common cause of your type problem. Doug