I have an electric fireplace in my 5'er. It was factory installed and works great. It is a forced air electric heater and does a nice job. Right now I'm on a host site with electricity included so I am heating with electric 99% of the time. I have run the propane furnace a couple of nights when the temp dropped to 13 degrees to keep heat to my tanks, etc. Even then I set the thermostat to 55 to keep the furnace running at a minimum level.
The electric fireplace I have:
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof