Your RV heater may be blowing cold air for many reasons depending on what type of RV heater it is and what condition it is in. Pressure decreases as the temperature decreases and the propane gas condense. It might just be so cold that the pressure has decreased enough that the furnace fails to fire, and thus cold air is blown in a propane furnace. A diesel furnace may blow cold if there is too much carbon buildup. Another common reason that a furnace may blow cold is low battery voltage disrupting the heating process. A final common reason is a safety component in the heater called a ‘sail switch’ becoming struck.