Coating/Covering plywood suggestions for my build

[Neal]

My plywood is bare. I never smelled any fumes. Sometimes over thinking is the enemy. If you go full time I guarantee you your setup will not look the same in 6 months.

[Bster13]

I realize my sister will change over time, but I definitely don't want to leave it there as I want to be as still as possible to match the black interior. The other thing about leaving it fair that worries me is what if I spill water inside? I guess that could lead to swelling of the plywood?

[BinDerSmokDat]

Moisture is your enemy. Whether from spills or from leaks or form condensation, water will find you and your unsealed surfaces.
I sealed every surface of every piece of wood in my build with urethane. Top, bottom and all four edges of every sheet and stick, with surfaces sealed BEFORE assembly.
That is what you do in boats.

That might seem excessive but consider that not only has this saved me from a few spills and a minor leak, it also means my wood is more stable to changes in humidity, less expansion/contraction and less ambient humidity in the vehicle.
The less your interior and belongings can absorb humidity, the less change of air you need to reduce ambient humidity in your rig.

[TheHobo]

Your best long term option would be to use a polyurethane, you can get tinted ones even black. Carpet won’t prevent water damage just hide it.