Won't put out anywhere near what the furnace will and won't be enough in colder weather and doesn't provide even heating throughout. As well, electric heater output varies as the square of the voltage, so by the time it's down to 105 volts, you've lost 1/4 of it's rating. It can get down that low sometimes as we have found out (in temps low enough to need heat).
Depending on the size of a camper, the amount of heat may be woefully inadequate in colder temps. If you have an enclosed underbelly, the floor will feel cold without the furnace running.
Then there's the question of what to do for load management if you have a 30 amp service. If you have 50 amp service that's great but it's often hard to find a 50 amp site in a CG (due to NEC min. requirements).
They're aesthetically nice and are a lot quieter than a furnace.
I installed an electric heater in each of our 3 rooms/areas totaling 1750 watts and the heat is very even in our 29' TT and adequate to down to about 38F temp. If it's just an electric alternate to the furnace you want, a 1500 watt residential kickspace heater like what I did in the photo would work with either a built-in thermostat or a remote one. Not sure what they charge for a fireplace option. I might even question the quality of the ones they install - maybe better to install a good one yourself?
If it's more the look and feel of a fireplace that's what you want, go for it.