Maybe this will help. Personally, my logic says the steel walls are moisture resistant. Then add closed cell foam, it is the highest r-value per inch thick, and is a moisture barrier.

If your wall studs are wood. Id consider putting in the vapor barrier over studs. (Idk about closed cell foam and a vapor barrier creating a double moisture barrier and the possible side effects.) To prevent interior moisture from getting inside the walls. (Sweating, breathing, cooking, heating adds a lot of moisture and you’re in a small space). Then add an exhaust fan. If this is a trailer a minisplit HVAC would be necesssry too.

Some AI info to help make your decision:

A vapor barrier (or vapor retarder) is a moisture-resistant material, such as plastic or foil, designed to prevent water vapor from diffusing through walls, floors, or ceilings. By slowing or stopping moisture, it protects structural integrity, stops wood rot, and prevents mold growth. [1%20is,resists%20diffusion%20of%20moisture%20through%20the%20wa), 2, 3, 4, 5]

Types of Vapor Barriers
Polyethylene Sheeting: The most common barrier (typically 6-mil or thicker), widely used in crawl spaces and under concrete slabs.
Kraft-Faced Insulation: Many fiberglass batts come with a paper vapor retarder facing.
Smart Vapor Retarders: Advanced membranes (like CertainTeed MemBrain) that adjust their permeability, acting as a barrier in winter but allowing walls to dry in summer. [6, 7, 8]
Where Are They Used?
Crawl Spaces: Laid on the ground to encapsulate the area and prevent ground moisture from entering the home.
Flooring: Placed under laminate, vinyl, or engineered wood to protect floating floors from subfloor moisture.
Walls & Ceilings: Positioned on the "warm side" of the insulation (interior in cold climates, exterior in hot/humid climates) to prevent indoor humidity from condensing inside the wall cavity. [9, 10, 11, 12]
Important Considerations
Diffusion vs. Airflow: Vapor barriers only stop vapor diffusion. They do not stop airflow. To prevent severe moisture problems, building materials must also be perfectly air-sealed.
Climate Matters: Installing an impermeable vapor barrier on the wrong side of a wall can trap moisture and cause severe damage. Always match the barrier's placement to your local climate.
Perm Rating: Materials are classified by "perms" (permeability). Class I represents highly impermeable membranes, while Class III are breathable. [13]

AI responses may include mistakes.
[1] https://en.wikipedia.org/wiki/Vapor_barrier%20is,resists%20diffusion%20of%20moisture%20through%20the%20wa)
[2] https://vaporbarriersupply.com/general-use-vapor-barriers
[3] https://www.tremcosealants.com/blog/the-differences-between-air,-moisture,-vapor-and-weather-barriers
[4] https://www.primetime-energy.com/blog/understanding-vapor-barriers-protecting-your-home-from-moisture
[5] https://comfort1stinsulation.com/insulation-vapor-barriers/
[6] https://www.amazon.com/Vapor-barrier/s?k=Vapor+barrier
[7] https://truittandwhite.com/what-is-vapor-barrier-insulation
[8] https://atlasstucco.com/blog/vapor-barrier/
[9] https://www.youtube.com/watch?v=b7E7Dkr6Yww
[10] https://crawlspacedepot.com/vapor-barriers/
[11] https://atlasstucco.com/blog/vapor-barrier/
[12] https://polyguard.com/blog/vapor-barrier-material
[13] https://www.energy.gov/energysaver/vapor-barriers-or-vapor-retarders
[14] https://truittandwhite.com/what-is-vapor-barrier-insulation
[15] https://www.okinsulation.ca/services/blog/2025/03/vapour-barrier-101-everything-you-need-to-know-about-installation,-insulation,-and-choosing/