Get 1" foam board, 3M spray adhesive, canned foam, some Liquid Nails or Lock-tite caulk adhesive, a calk gun, scissors, and some of the silver Reflectix bubble pack insulation for Lowe's. Also get some of the silver sealing tape which will be found over near the paints, but don't get the foam-backed tape for plumbing, just the silver foil tape with no backing. You'll need way more of everything than you think you will, and it'll take about 5 days to do ir properly.
Insulated the walls, ceiling, sliding and rear doors. Be careful of the driver and pasenger doors, cause it's (a) more trouble to insualte than it's worth and (b) you'll end up screwing the window track up if you try, so don't.
Cut the foam board to fit as many of the voids as possible. Use the canned foam or the Lock-tite to hold the foam boards in place. Then fill in the rest of the voids with canned foam. The use the spray adhesive and the Loci-tite to glue the Reflectix to the walls, ceilings and doors. You want no exposed metal, or as little as possible, since the metal will act as a heat sink. Make sure you seal every Reflectix seam with the foil tape.
Another option is to take it to some place where they'll blow in the foam insualtion into the walls, ceiling and door panel voids. Should run you about $500 for that, which is about what you'll pay to do that part of it yourself, anwyay. After the foam is sprayed in, do the Reflectix thing.
Sprinter
Ford before insulation, and after (think Jiffy Pop)