Adding a fireplace (electric fireplace heater)??

Yes, you want to make sure that the receptacle and connected wiring is rated for the load of the fireplace. As some have found, the cheap receptacles installed in most RV's fail when pushed to their limit. If it was me, I'd install a separate box w/conventional residential 15 amp receptacle, making sure that the wire is capable of the draw of the fireplace.
Adding a new branch circuit also has merit but some RV load centers, typically, are low in open breaker spaces.