I have thought about using something like lizard skin under the bus to help create a bit of a barrier - I have seen that it was done on some buses - but can't find anyone who has tried it, but I can sure get opinions about it. My floor has two coats of rustoleum, a layer of water/ice barrier, reflectix, 1/2 plywood, 4 coats of paint, vinyl planks.
My floor will generally be close to outside temperature - my walls and ceiling are factory insulated and do pretty well.
For example: It's 34 degrees out right now (I'm in Montana) - my floor is at 51 degrees (including my stairwell - which is a single piece of sheet metal, not insulated) - the carpet areas on my floor are at 61 degrees. My ceiling is 75 in the front of the bus and 65 in the back (I have a wood stove in the front burning). The ribs at the ceiling in the back are at 55.
My walls are at 63 degrees.
The outside metal of the bus is at 41 degrees - but will likely cool quickly as the sun is down.
I am not sure that helps any one - it's just what it is in my bus.
I never have a consistent temperature on the bus. The back is always colder in the winter and warmer in the summer. The doorway is drafty. The windows are cold and drafty - except in the summer, they get really hot.
But, it's a bus. And it's home. And I like the variety.