The skin of the trailer is water resistant. There is a lot of controversy as to how well you want to seal things. The trailer walls will sweat and there will be leaks from time to time. Where is the water going to go? It should end up in the C-channel at the bottom of the walls. You want to prevent large pockets of air that can convect heat but you also want things to breathe to some extent. I taped all seams of RMAX ridged foam but left the bottom edge unsealed. This prevents air from moving around but leaves a place for water to go. At the bottom of the C-channel, I put in drains so water has a place to go other than the floor. The reflectix stuff and similar products work best when there is an air gap on both sides of it. You could use it as a top layer to foam insulation. The RMAX has reflective aluminum on one side and painted aluminum on the other side so it also acts like a radiation barrier as well. All of these insulating materials that are not radiation barrier materials, use air to insulate by separating the air into little pockets where it can't transfer heat well from convection. The radiation barriers work by reflecting heat back where it came from and also keeping it from radiating to the cold side. If something touches the surface, then it does not work as well because now conduction is shorting out the radiation barrier. You don’t have enough room in the walls of an Airstream to have much of an air gap on both sides. Also the radiation barriers only work when they stay clean and shiny.
Boric acid is used to preserve wood and fabrics and it will wash out and I am not sure what the acid will do to the metal. It is also a flame retardant. Stuff that is preserved this way will develop that old canvas tent smell.