Here is something else to consider:In a small trailer, the people are a primary heat source (98.7 degrees F), But the people are also a massive humidifier, breathing and perspiring especially when it's cold and the inside is warmed by a heater.
As Cindy and others have noted, that moisture is going to find its way into the walls where ever there is a path, no matter how small. The metal skin (aluminum or steel) is going to be cold and cause that water vapor to turn to liquid water on the walls. BUT, if the steel skin is waterproofed, it's not going to rust. Then if inert, non-absorbent foam is stuck to it without an air gap, there's not going to be any condensation. If the skin is aluminum and not waterproofed, the condensation is not going to rust the metal, but streams of water are going to form on it and run down to the wood floor and rot it and the interior wood paneling closest to the floor. My trailer is aluminum and has tight block foam insulation. No matter. There are still micro-air gaps. Condensation forms in the walls and ceiling and then warps and stains the wood paneling close to the floor. Dry rot is on the way. I blew it by not spray foaming the whole thing.
R-Value is far less meaningful in a small trailer than in a house. Why? People/Occupants are a considerable heat source PLUS most of us can only get 1" of insulation in between the interior and the exterior. So don't weight R-Value over preventing condensation. Any R-Value over 3 will do!
What would be the BEST we could do to make this insulation situation right? It would be to either spray foam the walls and ceiling OR to waterproof the interior metal (skin and ribs) with roofing tar or elastomeric roof coating and then to adhere block foam to that while it's still "wet."
The beauty of spray foam is that it's Gorilla Glue (polyurethane!) that expands a lot, and it gets all over everything as a liquid before it expands, waterproofing whatever it has coated. It's messy and well worth it!
Cotton (or other sponges) is the worst possible alternative to this.
FWIW.

My goal...
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...is to live in a trailer.