Another thing you referred to is heating up! When the unit is cold the air will pass over the heat exchanger, being the unit is cold the heat exchanger cools off and the fan will stop. Furnace reheats and fan comes back on , this will contuine till the RV is heated up and will take a while depending on how cold it is inside and outside. Should the burner be cutting on/off blocked vents are a common cause or varmits built a nest in the ducts or never hooked up.
Make sure all vents are clear, and there is no filter in the furnace.