Relax, a number of professional caravan and campers have some hardboard in their construction – it’s not as good as ply, but it’s no disaster. As long as it don’t suffer from direct water soaking from drips and leaks it’ll last for a few years, I expect, especially if it’s painted. The paint will tend to seal the board – especially so if you make sure you get the paint onto all the edges to stop the damp creeping in these unprotected areas.
The walls will tend to be held more firmly by whatever furniture you choose to build inside as well, although the ceiling might require some extra screws and support as gravity will be against you. But as you say, see how it goes and replace it if you find it gets unsightly.
There’s nothing wrong with following the herd – with grey carpet inside and having those crayon swiggle graphics outside – if that’s what you want of course. But I always approach a conversion as an opportunity to do my own ‘Grand Design’ – as you may never get to build a house from scratch, but on a van you get to design the bedroom / kitchen / bathroom etc entirely yourself. On a personal note for instance, I use pine tongue-&-groove cladding for the ceiling – as it’s strong, cheap and easy to work with. It happily takes the curves around the barreling of the sides too. A bit ‘old fashioned’ I’m told, but used as part of the design it looks great. I did a VW T25 a few years ago with one, then visited a local beach and collected some shells / netting / rope and driftwood that I used as a mirror surround. Everyone loved the simple ‘wooden beach hut’ look and surfers were fighting each other to buy it off me…
Enjoy your building