Just a thought, but I would bet Owens Corning has a tech support phone. I'd call them and tell them what you're up to and see what they say about water resistance etc. of their panels.
Other than that I would make sure that there held in place with some strong adhesive. I'd also think some type of mechanical fastener like fender washers/screws would not be a bad idea too. One of those panels coming loose, flying out from under the camper on the road, might not be fun for the vehicle behind you. Especially if it gets airborne.
I like you're recycling the original camper parts and am enjoying your build.