Of the various types of insulation commonly used in skoolies, polyurethane closed cell spray foam generally provides the best R value per inch. It also works very well to prevent air infiltration.
I have seen one bus that they put 3" of foam in for an R value of about 21.
I put about 1-1/2" in my first bus and was happy with it except in more extreme temperatures. I am going to try and manage to get 2" in my new bus.