A fireplace doesn't mean your RV has a heat pump. A heat pump will be in the a/c, but not all a/c's that have a heat setting will be a heat pump. You could have a heat strip, also known as a chill strip mounted in the a/c.
Just to try to make it simple to understand, an a/c heat pump is made with valves that allow the reversal of flow of the refrigerant. When the a/c is in cooling mode the refrigerant absorbs the heat from inside the RV and exchanges it outdoors. When set to heat pump the process is reversed and heat is absorbed from outdoors and exchanged indoors. Heat pumps will not work well, or at all, when the outdoor temperature is below 40 degrees.
Google "what is the difference between an air conditioner and heat pump" for a more detail explanation. This link explains it pretty well:
http://www.heat-pump-pro.com/heatpum...nditioner.html