Good afternoon! I've used marine plywood before, but it was industrial grade, not BS-1088 ... whew ... just found Hydro-Tek BS-1088 online for $137/sheet in 18mm (23/32") thickness.

Are you planning on fiberglassing your exterior?
You probably already know this, but a higher grade of marine plywood doesn't necessarily mean more strength or durability, unless you're talking bending strength. It's prettier, more dimensionally stable, etc., but almost any marine plywood will have a decent WBP (water boil proof) adhesive. Some grades may have a fungicide added. If you're already planning to skin your camper in metal, is it worth spending the extra $$$ for the structural sheathing? Not saying it isn't! Just something you have to consider.
If you're building a woody, you could always purchase 3/4-inch X-rated sheathing for the walls and 1/4-inch X-rated sheathing for the roof, and then laminate 4mm Hydro-Tek plywood on top. That way, you get the looks, stability and weather-resistance of Hydro-Tek, but without the dire cost
