I would suggest you spend a couple of weeks researching insulation and R value interpretation. Understanding what the goal of insulation is attempting to accomplish is also important. For a camper or home, it is to slow the transfer of temperature for one side to the other. Specialty insulations like Ceramic fiber are more about handling high temps with out being destroyed than having increase insulating value. What makes for a good insulation is the ability to create multiple air gaps in the given space to limited air movement. Closed cell foam is great but varies base on the size of air bubbles it creates as it cures. Many manufactures used Ensolite before Reflectix was available. At the 1/4" thickness the difference is almost negligible but if you go to 1", the Ensolite is far superior, cost and weight
not being a factor. In the end, thickness matters and just about all insulations have minimal impact in gaps less than an a couple of inches.
Real world example I also have an 1977 Airstream that I have been completely redoing and I have three different types of insulation in it. The wall gap through out is 1 1/2" and I have fiberglass, rockwool, and synthetic batting with aluminum backing (HVAC ducting insulation). On a sunny 95 degree day, the surface interior panel temperatures were within 1 degree degree of each other at 87 degrees.
Good luck with what every you choose