Keep in mind that even if they are 4-season RVs, they will still not be as well insulated as a house.
According to where you will be wintering, you may have to also enclose underneath the RV by the use of some type of insulating material, you may have to use heat tape on the water source, supplement your heat with electric heater and you may need to add a larger propane tank.
My son has been full-timing in Kentucky for a couple of years so he has discovered most of these things and is usually pretty comfortable. That being said, he had some pretty uncomfortable days during this past week, but the temps that the whole country had was pretty uncomfortable.