Adzel SuperLite is a premium RV wall panel substrate used by high-end RV manufacturers like Coachmen. I haven't used it yet, but I can't wait to try. Better impact resistance, sound absorption, moisture resistance, thermal insulation, etc., than plywood.
Fair warning: It's expensive. $50 for a 1/8-inch 4x8 sheet + shipping. So maybe not something for most DIY builders. Also tough to glue because it's LSE.
Anyway, it's a bear to find. You have to buy 200+ sheets direct from the manufacturer. I've only found one other supplier:
http://www.mirageinc.com/superlite-rv-w ... white-44mmWhere you can buy small orders. White color only.
I don't usually give away my suppliers

but I'm not too worried about this one. Adzel is a specialty item, really only useful for ultra lightweight double-wall construction. But if you're building that sort of wall, and want the best, then I hope this helps.