I have the ensolite in my 19' '84. I have two electric heaters, one is an electric "fireplace" with no thermostat, the other is thermostat controlled. At 30*, with only the fireplace one running on the lowest setting, it cooked us out, and we had to switch to the thermostat controlled one. I set the fireplace on low, and the thermostat one on 65, and worked in a t-shirt in it, during temps in the teens.
I wouldn't worry about wall covering.