I used (1 inch thick, I think) foam board panels for the larger void spaces on the side body and ceiling. Glued them with Loc-Tite adhesive in one of those big caulk gun tubes. Filled in around that and in the other voids with Great Stuff canned foam. Then glued up Reflectix bubble pack insulation to cover everything, making sure to use the silver foil tape to seal all the seams.
It is tempting, and a lot quicker, to pay a professional the roughly $500 to spray foam on the walls and ceiling, but you run the very real risk of rippled body panels. They'll do a whole side of the van very quickly, and as it cures it generates a lot of heat, which will make the body panels prone to rippling. If you have a professional do it, express those concerns first.
Do not use spray foam on the doors, especially the rear doors. The foam will block water drainage and will cause the doors to rust. Trust me on this. Or ask any body shop guy.
I think it took me 5 days to finish the insulation and Reflectix. It's not something you really want to rush. You're gonna be living in it, after all.