Been a while, still building my camper out - here's some update photos -
Cabover

Bathroom (you can see where sink will go)

Dining Area / Bedroom 2

PET walls lookin' nice with the shadows
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We used aluminum tubing to support our dinette area, kitchen as well as bathroom walls.
These sections are attached with rivets and then fiberglassed into the body to create a strong bond that will support the constant vibrations in the shell.
Walls
Bathroom walls are ACM aluminum backed polypropylene sheet, typically used for sign making. They're a bit sturdier, cut easily like the PET panels and have great moisture resistance.
Ceilings, cabover and non cabinet walls are PET panel material made from recycled plastic water bottles. Feels a bit like felt. These materials were left over from another build and worked perfectly in this application. These help reduce sound, add a tiny bit of insulate value, they're easy to cut and place and they look nice! We used a backing piece to allow us to cut screw flaps which would conceal the attachment points but make them accessible later.
Cabinets and wood furniture are going in next, will share those photos as soon as I have them.