I have skirted places with the Hadrie backer board (not the cement stuff) used for tile under-layment it comes in 3'X5' (1/4" and 1/2" thick) sheets at about $10 a sheet and you could get 2- 1 1/2' rips out of 1 sheet. Its the same stuff that the "cement" wood looking siding is made out of. Very strong, water/mold resistant, bugs don't like it, animals cant chew or burrow through it, and once its painted looks just like plywood. Since tile isn't going on it you can use galvanized ring shank nails and put it up real quick; to cut it you use a utility knife score it about 3-5 times and then snap it like drywall.
use the weatherized wood (green board) for the ground to trailer posts and nail away. That wood will take years to rot. Especially if you use a brick for the wood to sit on and put the cut end up.
A rainy day camping is better than any sunny day at work
2004 F250 V10 SD Xcab 2011 Wildwood T26BHXL