Softwood timber batten goes on the edges & rest of interior body. Glue & seal everything with polyurethane sealant/adhesive. Google that. Various brand names Sikaflex512, Tigerseal etc.
If you mean the plastic infill to cover screws. It's called Herzim strip. Any caravan shop should stock it.
If you have any converters anywhere near you....That also sell larger motorhomes second hand or whatever I would go and browse their stock...looking like you were about to buy?....see how they do things. I wish you good luck with this, IT IS MORE than I would dare set out to do.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets