My ceiling and walls have 1.5" square tubing in them, I stuffed polyisocyanurate between the tubes and added another 1/2" over that for a total of 2".
With 1/4" panelling on that, I only lost a total of 3/4". Some people have more, some less. It depends on where you're going to spend the winter (or summer for that matter).
An MCI 102 C-3 has about the same headroom (maybe more) and huge baggage bays for water and holding tanks etc.
Ed