Pulling the walls and ceiling also allow you to inspect for leaks and remove potential mold in the insulation.
weather you are building for cold or hot weather, insulation is paramount to be cool or warm.
As O1marc stated, every inch matters when you have such a limited amount of square footage.
Im 6'1" and my roof is 6'5" from steel floor to ceiling ribs, so good quality polyiso floor insulation and ceiling insulation was the best choice for me. Ive got 1 inch of floor insulation and 5/8" OSB for the subfloor. Then 1.5" of ceiling foam and 1/4" of ceiling planks. This should leave me just over 6 feet to walk down the middle of the bus. The walls will have 3" of insulation. Planning for 4 seasons use.