I replaced about 1/3 of the Coroplast on our trailer. I used 4 x 8 sheets which I bought at a sign shop. Prices varied wildly, so shop around. The best price I found was about $16 per sheet. Stainless steel fender washers and hex-head screws worked well for attaching the Coroplast. I did have to add some structure (2 x 3's) in order to have something to screw into. Be sure to overlap joints so it won't catch the wind. Working around tanks and gas lines were the biggest headaches. You can't have a joint under a tank.
I would also recommend adding insulation while it's open. I glued foamboard to the underside of the floor.
