Got a 18 salem 27dbk trailer. Using it for full time living.

x-2 on the coroplast, plywood will rot after a couple of years coroplast should last longer, insulate better and be lighter. As far as attaching it, get some large fender washers and use self tapping screws with the big washers to spread the stress. You could even add some metal strips to bridge from one frame member to the next.