I am so glad that you took the time to call the company and talk to them about the specific insulation product you are using. That was a very wise decision.
It sounds like this product has an actual metal foil on the surface versus the shiny mylar plastic used on Reflectix. I like that!!! But even with a true metal the radiant barrier does need to have an air gap next to it on the side that is towards the heat to achieve the maximum stated R Values. But as they said there is some benefit to it despite the lack of air gap.
For those of you who have Reflectix it is important to note that there is a difference between being a radiant barrier and a reflective barrier. Metal is a radiant barrier and if polished can additionally be a reflective barrier. It performs best when it is shiny since you get both benefits. Reflectix is a reflective barrier and it has no significant radiant barrier capabilities. So if you dull the surface of the mylar plastic with glue it loses the ability to reflect heat. Foam is more effective in R Value than the dead air space of a double bubble layer.
The true metal facing on the foam board will help deaden sound too. Reflectix does not have sound deadening properties. Closed cell foam does have sound reducing properties. Sound deadening is nice to have in small trailers as rain can just about drive you nuts going on for hours if it is coming down hard and you are a very light
sleeper. Plus helping with those neighbors with generators that are running for hours on end. So when choosing an insulation you get extra value if your choice does help reduce the sounds.
Vinyl coated headliner/seat topper is available from various sources, it is used on boats. It is a 3/16" thick closed cell foam coated with a layer of colored vinyl. Here is one supplier for it. Foam Back Marine Vinyl by the Running Yard 54 Inches Wide
That material is one of my lucky leftover finds I got and I have enough of it for my Campster
ceiling. In the flat areas I will first adhere the thicker layer of closed cell foam (that was also a free leftovers find), then I will top it with the vinyl coated foam layer for the finished surface of the ceiling. I don't need to put the thicker foam over the molded in stiffener beams in the Campster
ceiling as they already have foam inside of them. The foam created the shape and fiberglass cloth and resin over the foam provides the strength. That is a structural method of stiffening areas that is also found in fiberglass boats.
If fire rated non toxic fumes materials is a big concern to you that issue can be solved by purchasing insulating and upholstery materials from the aircraft interiors industry. The FAA requires certain standards of non toxic fumes in case of fires. If you want to find some just use the keywords "FAA approved" tagged onto the kind of product you are looking for.
One of my personal tricks is to apply the aluminum tape materials to the backside of wall panels or to the opposite face of foam insulation board that has no reflective surface. There are many places where you can have hot days and cold nights so you are trying to keep heat in as well as keep it out at different times of the day. But I will post more about that in my personal modification thread when I reach the part of my project where I begin installing insulation on the upright walls.