Well I did say if its not going to get properly wet. Its for a bed, not to make the shelving that holds dripping wet boards.
No need to screw into end grain when plopping it on a frame for a bed.
For some people, $10 vs $60 a sheet might help,so I threw it out there. Never said it was the gold standard, just a cheap option if its not going to get wet wet
10mm painted MDF in mine. Been there for years. No signs of water absorption/swelling/warping. Ive made sure its well supported (i sleep on it) and i do make a habit of keeping the van as dry as possible, but my boards live in there. I don't pull them out after a sail.
It can most certainly absorb moisture. But done the right way, and with a little care it's a viable budget alternative.
I use Yellow Tongue flooring particleboard in mine. It's wax impregnated and quite water resistant.
www.bunnings.com.au/our-range