Because I was using framed construction I used the thinnest facing I could readily obtain, ie 3 ply lessi have used the vohringer boards etc , great if you want a van that looks just like all the other vans , but far thicker than needed in most situations . nice and easy to use mind,but stupidly expensive . i like 6mm birch because it's got enough plies that it'll hold a screw pretty well. the 60cm square boards i use to fill in between my seats to make the bed are 9mm birch ,which i was a bit unsure of ,but are easily strong enough for the job.
than 4mm thick. Most of the framing and battening was 12mm or 15mm so plenty to screw fix to.
I also made most cupboards as panel units with plastic corner fixing blocks, the idea was to make
it practical to remove items, & change the layout if required in the future.