I have been talking to people about their camper van conversions for more than 10 years. Each is different. Many include lots of self expression and passion. The one thing they have in common is the need for thermal and acoustical insulation and "Oh, I've heard the moisture we produce while sleeping condenses on the cold sheet metal, gets trapped in the insulation and causes rust".
This has been discussed in reference to VW buses, heavy duty Ford and GM vans and now the new generation of Sprinters, Transits and NVs. I didn't have a solution but felt strongly that bubble wrap and fiberglass insulation weren't it. Denim insulation has problems with moisture. I could see why the treatments being suggested wouldn't meet all requirements but wasn't sure what would.
Turns out I was getting in my own way on this. My focus has always been noise reduction. I have advocated using vibration damper on just 25% of a panel's surface instead of the 100%+ promoted by others. 25% coverage is the standard in most applications outside of aftermarket automotive. It does the job and makes future sheet metal repair much easier. Our signature product, CLD Tiles™, is optimized for this exact use.
What if vibration damping isn't the only concern? I remembered that a material using the same components as a typical constrained layer vibration damper - aluminum foil facing with butyl adhesive - was used to protect pipelines in extremely harsh conditions. I worked with the manufacturer of our butyl products to "stretch" our CLD Tiles™ into 12"x24" CLD Sheets. Same adhesive, 4 mil instead of 8 mil foil, Slightly less effective as a vibration damper which doesn't matter since we will be covering the entire panel. Still an amazingly good vibration damper. If there weren't CLD Tiles™, I'd use CLD Sheets for all of my noise reduction work. More interesting for use in a camper van, they have outstanding corrosion prevention characteristics and can be removed if needed.
I put a 6" square of CLD Sheet in the middle of a 12" square piece of 24 Gauge Cold Rolled Steel. I exposed it to salt water for 3 days and peeled off the CLD Sheet. You want to completely cover all of the wall and roof interior surfaces that will be in direct contact with the thermal/acoustical treatments. Vibration damping and corrosion protection done.
Blocking and Absorbing Sound / Thermal Insulation
There are two more steps in the process. We use mass loaded vinyl (MLV) for all of our noise reduction projects. MLV is a limp barrier that weighs 1 lb/ft². It's purpose is to block sound traveling through the air. We use MLV below the glass line in most vehicles. This is where most noise enters a vehicle so nothing is sacrificed. The glass is always the weak link, so there is no point going higher in the vehicle.
MLV needs to be installed in as close to a continuous layer as you can get. Sound will bend around openings in barriers and we want to provide the fewest possible opportunities for it to do so, without interfering with vehicle operation. A van will gain some additional traffic noise reduction by covering the wall from floor to ceiling, but I would not do it because of the added weight and expense for a minor gain. Stick to the floor, wheel wells and lower walls and doors.
MLV should not be in direct contact with the vehicle to prevent vibrations from traveling into the MLV and because it functions better with an airspace. We almost always use closed cell foam (CCF) as the decoupling layer. This is the Floor of a Sprinter. MLV is visible. The layer of CCF is underneat the MLV.
We use Hydrophobic Melamine Foam (HMF) to absorb sound and for thermal insulation. It is very good at both, won't absorb water and won't burn. Fantastic Material. Here's where your interior finish plans come into play. You'll want to fully fill the voids in the walls and roof with HMF but it should be protected. You'll want a headliner for the roof and some kind of covering for the walls that are treated.
Sprinter Van Lower Wall filled with Hydrophobic Melamine Foan, to be covered with MLV and the stock trim panel next and a wheel housing covered in CCF and MLV, Both areas treated with CLD Sheets first.
Sprinter lower wall filled with HMF. We taped a sheet of MLV in place temporarily until we replaced the stock trim panel. Trim panel screws secure both trim panel and MLV. In this configuration the HMF acts as the decouple for the MLV. You will have to adapt this to your finish plans. An alternative configuration would be to hang the MLV on the CLD Sheets applied to the outer vehicle skin using the High Temperature, Vinyl Compatible Velcro Strips we use for most vehicles, using this technique: Attach MLV and CCF to the Vehicle Then fill the remaining void with HMF and finish the interior as planned. There will be more breaks in the MLV layer this way but it should still be very effective.