I'm currently building my #1 (TPCE) using a hybrid foamie construction method. My intended use is for two track forest roads and some rough conditions. Not really rock crawling, and I don't expect to be scratching my way through tight brush and branches, but light to moderate off road use. Jarring and bouncing.
My walls are 1-1/2 thk foam and will get 5 mm (about 3/16 thk) ply inner skin adhered to the foam with contact adhesive. There is minimal wood framing in the walls, just around the doors and galley wall edges, and some surface blocking imbedded here and there to screw the cabinets, bulkhead and fixtures into. The outer skin will be a wrap of either canvas and paint, or maybe glass fabric reinforced epoxy (haven't decided yet). No outer plywood. Fully welded box tube framed trailer.
The best advice is to make a small test sample and decided for yourself what method gives you the strength that you think you need. From what I have read, on a traditional built up wall, 1/4 outer is more than plenty. I can't recall ever reading someone say that they wished they had used 1/2 instead of 1/4, but I have read many times where people have said that they wished they had used 1/4 instead of 1/2. If you are going to glass reinforce, 1/4 inch ply must surely be more than adequate.
The strength is in the composition of the whole structure, not any one single piece. Build light and glue everything to each other.
My Build: The Poet Creek Express
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"TnTTT ORIGINAL 200A LANTERN CLUB = "The 200A Gang"Green Lantern Corpsmen