I personally think an electric heater is a great idea. Furnaces are very noisy. We used ours once and couldn't take the noise so I installed a 1KW, semi-recessed, hard-wired heater in the ceiling near the A/C unit as in this photo. It runs on a low voltage thermostat on the wall that looks exactly like the one for the furnace. All wiring is hidden in the walls and ceiling and looks like a factory install. Our trailer is on the smaller side at 20' and is a single area except for the bathroom. It has worked very
well. Only thing is that I wish I had installed a 1500W unit instead as the one I used cycles on and off more often than I'd like. It still makes some noise but is waay quieter than the furnace and we can sleep at night. This is my favorite mod. I've done to our TT.
I guess if you get charged a high rate at a c/g, propane may be a better option. At least on electric though, you never need to worry about draining the tanks dry when you least want it to happen and you don't have to drive around lokking for a place for a fill, esp. if it's dark and/or cold out.
If you wanted to retrofit with a heater(s) installed in the ceiling, your camper would have to have fiberglass insulation to be able to fish wires in.
If you have a 30 amp service, you are pretty much restricted to using a max. 1500 watt heater so that you can still use other things. If you have a 50 amp service you can install more or higher wattage heaters. In a larger camper with several rooms and a 50 amp service, you'd want an electric in each divided space.
I think if you want to use an elctric heater(s) in cold weather, unless you have a well-insulated or 4-season camper with more/better insulation, you won't get enough heat out of an electric heater. But I doubt you would remove the furnace, as it'd still be there for the times when it gets cold and you need more heat.
Radiant heaters heat people and objects and not air. They are in the order of only 10% more efficient than space heaters. I don't see them working well in an RV since the ceilings are too low. You'll often see these heaters in warehouses and shops with high ceilings and in spots where people work a lot in the same location like at a counter or workbench.
I don't see how electric radiant heating cables would would in an RV. These normally need to be installed in a mortar bed. I'm not aware of any electric ones that work on or below a wood subfloor unless there's something new out there.
If you are using plug-in portable heaters, especially ceramic types, you need to be careful where you place them so as not to potentially cause a fire. The oil-filled heaters are quite safe but bulky. There is a retrofit kit available to attach to a furnce, but is very expensive and from what I read elsewhere, it's questionable if you'd reach a payback point to cover the approx. installation cost of $1500. Plus the kit is not a DIY install.
And if you are going to use portable plug-in heaters you need to be careful of what recept. you plug it in to, that there are no "dysfunctional" wiring connections at the recept. you are plugging into or downstream recepts., and never hide the cords under carpeting, dog beds etc. as the cords can overheat. It's also not good to let the heaters run non-stop as they aren't normally designed for 100% duty cycle. You can find posts/threads on this forum on all this.
What I think would be cool is an underfloor hot water radiant heating system like used in homes (we have it). This would be a serious retrofit though, if even possible at all, plus you'd need space for a boiler of some sort, circ. pump(s), valves, etc.
Anyway, that's my 2 cents for the last day of 2012.