Yeah, I used to be so sure you could just measure once and cut... then I built my first cabinet for the curved wall behind the driver's seat. Nothing fit, it was a total mess. Now I make a cardboard template for every single weird angle, no matter how simple it looks. It takes forever but it saves so much wood and headache.
Van geometry has its own set of rules, apparently. Every measurement comes with a built in margin for error, usually in the wrong direction. Just cut everything twice and hope for the best.
Forget the walls, the real trap is the floor to ceiling curve. You think you've got 6 feet of headroom, but try standing where the wall meets the roof. Suddenly you're buying a bulkhead of foam to fill a triangular gap you never planned for.