Any plywood will swell if moisture permeates the ends so careful sealing and cabinet design are key, but most of all it is important to note how rare your failure was. I'm on lots of camper forums and have never heard once of a porta potty or cassette toilet seam splitting. Sure, some folks have had spills but your issue is exceedlingly rare.
Many SMB's here, made of particle board, still last for years with average care. It's usually the T-molded edges that start giving way and certainly bottoms of cabinet faces that are often sitting on old carpet in old vans. Especially near the doors or under a window. Once you have a small leak things get ugly fast.
The go to for most of us in cabinet builds is birch. It's light and strong and pretty easy to work with. I guess I've done 4 or more vans in stained birch ply. There are many of these builds, mine included, in the home-made/ground up build section. Some custom DIY builds, some SMB rebuilds.
Another thing that is noteworthy is that very few Sportsmobiles have cassette toilets, not that the type really matters. Most late model vans have a removable porta potty. Or is that what you meant?
In your next build I would recommend a composting or separating toilet that doesn't hold large quantities of liquid. I am currently ditching my Thetford PP for a Joolca, if it ever arrives. It has separate chambers for urine and poo and you can use it as chemical or composting. If you haven't researched this it is the mixing of the two that causes the worst smells.
Top tier builds/rebuilds?
@REF
@Pschitt
@Boywonder
@Aarcaris
@teeots
@mgmetalworks
@scalf77
@saline
@bodhi1960
I'm sure I'm forgetting lots of folks