IMHO heating a skirted area doesn't do much. Well at least in my Momentum. The reason being is that there is insulation all along the bottom of my 5th wheel and all of the pipes and water tanks are inside of that insulated area and heated by the furnace.
The skirt creates a "dead air" space under the RV. Air movement facilitates temperature change, in other words, when the wind blows it cools things down. The skirt reduces that cooling effect. But heated air under the skirt does not transfer to heat inside the coach.
Another factor to consider is water. As we go through the fall/winter/spring cycle snow comes and then it goes. When it thaws the snow melt has to go somewhere, and it often collects under the RV and turns to ice. The year I tried a space heater under the RV I had to keep raising the plastic pads (you know, the yellow blocks we use for landing hear blocks) to get above the water line. Something about space heaters and water made me nervous.
So unless you have exposed water pipes under there I wouldn't use a space heater under the skirt.
In the RV I use three electric heaters, one is our fireplace, it is not super fantastic as a heater, but helps some. My wife keeps a little cube heater in the bathroom. She will close the door, crank up the cube heater, then go take her shower. Fortunately Utah is pretty low humidity so we don't get mold problems. It's like a sauna. Lastly we keep a
medium room heater in the kitchen, the lowest point in the living area. We have tried several different types, oil, box, cube, etc. But what I find is that they don't take too kindly to being bounced around in an RV. The one we use is fairly cheap, easy to replace, and gets the job done.
