I started with a d2 in my sprinter and moved to a d4 this past winter. No bunk wall full open but do have curtains I can use on each end of the bed. Ceiling is foam board covered reflectix with the gray panels that came on the walls. The walls are rolled insulation covered with bead board. The roll was stuffed in the bigger voids and spray foam in smaller ones.. nothing added to the sliding door and only a small amount to the rear. The d2 actually did well except on the coldest of nights then it might be 57 or 58 inside when I woke up. The d4 takes care of that problem.
I would never recommend trying to heat a full sprinter, uninsulated with even a d4, it's kinda like searching for the cheapest fuel then driving 75 miles an hour everywhere. It just defeats the purpose.