I'm very much not an expert, but having done a bit of research, if there's one thing I'd avoid myself it would be using units from domestic suppliers. The ply is massively thick. Instead I'd look at getting the thin ply that's used for marine/camper fit outs. If you're happy designing, you can get it cut to spec for not too much money, and do design where the dimensions of doors etc are the right scale for a van (rip off a pro design if you need to!)
Fron what I've seen of domestic kit based DIY, they look massively wrong in a van. Even as far as twee panel inlays, like some sort of mobile 1980s semi-detached barrett starter home. Yuck.
Sorry, just my tuppence worth.