The UK Camp Site , the number 1 camping and caravan resource on the web
                           

Wall boards and Panels

Sealants & adhesives

Trims and seals

Delamination glue

Screws Link_2 

Awning_rails

 

The rivets that hold bed box frame work to the caravan wall are called Peel Rivets .
A search on the internet will come up with quite a few suppliers .

25mm thick Polystyrene sheet can be found at B & Q , called  Jablite_board .
Also Here and Here for 20mm