To make the frame of the walls, we've used 75x45mm timber to form the main supports for the wall/cupboard/bed structure. The timber is both screwed and glued to the body of the van using Sikaflex, , and secured to the ply with little brackets. So it's super secure.
The non-structural parts are made using 38x25mm timber, which again is glued and screwed.
This is the main structure of the wall/shelf/cupboards...