yep, i wish i could find some FRP locally to try what you did. So far, only aluminum here at acceptable price.
do you remember how many tube of sikaflex 252 total did you used for your build?
Not sure but probably close to 50. This place had the best price at the time Sikaflex 252, White, One Component, Elastic Adhesive, 300ml Cartridge [SIKA-90915] : Silicones & Caulks - $11.49 EMI Supply, Inc
The place I purchased from will ship to you, but It will cost. FRP panels and sheets - Vetroresina LLC
You may want to check out other hardware stores for test purposes; Frp panels at Lowes.com: Search Results
Panels can be made from various materials like aluminum, tin, plywood, frp, etc. For camper builds, you obviously want light weight and thermal neutral.