This has been discussed here (at length I believe). Opinions vary. The van is made to have condensation form on its cold interior walls and then travel down to drains along the pinch weld at the bottom. Interiors of doors get lots of water in them as the seal on the roll-down windows is not perfect. All this is fine and rust resistance will allow it to continue for many years especially if the interior drys between condensation events. The problems occur when it cannot drain or is not allowed to air dry. Cotton, spun fiberglass, many rigid foam boards and many other hydrophilic materials keep the moisture from drying and hold it against the walls causing rust..... bad do not use. Other materials may block the drains things like misapplied spray foam, cotton, and shredded or poorly installed paper like labels...... bad do not do.
Hydrophobic spray foam, automotive Thinsulate, Polyisocyanurate foam boards and some other materials do not suffer this problem...... good use them.
Planning for water escape through the normal drains as these materials are installed is possible.
Vapor barrier is not a good idea anywhere in this system. The interior spaces in the walls must be able to dry as air exchange with the wet interior of the camper at night will happen, and air exchange with the dry interior during the day is going to be necessary.
Lastly... Condensation only happens when three things are present: 1. Air with vapor in it 2. A temperature below the dew point for that air and 3. A surface to condense on. If the dew point is INSIDE the dry insulation no condensation will occur...... good. This is why sprayed in foam works, non-moisture absorbing polyisocyanurate works, and hydrophobic Thinsulate works. BTW styrofoam bead board absorbs water like a sponge, blue (or pink) extruded polystyrene absorbs water quite well, fiberglas batts absorb water nicely too. Why go there?