I will never for the life of me understand people not insulating the floor, it works in both directions hot and cold.....
I work on the flightline radiant/reflective heat of concrete, and asphalt is ridiculous.
Not to be cynical, but if the person "elected" if you will to not insulate, whether their reasons made sense or not. What else is hiding under there??? For my own piece of mind I would tear it out and assess the metal, then insulate the floor and lay a subfloor.
Cold settles, you're in a metal box, and as others have mention the wind flow under the bus will cold soak the bottom of the bus. If efficiency and comfort are goals, then there is no such things as too much insulation, just a matter of how space are you willing to give up. The less effort it takes to warm and hold the warm the better.