That's why I think it's a plastic. It's got to be. If various trailer dealers are toting "custom high quality race trailers" the lining would have to resist that sort of thing. Metal sheeting is more expensive, incurs more weight, and therefore takes away from your payload. Just more conventional as a plastic in this case.
Aviz, don't feel bad, I'm _ this close to getting that white paperboard. It's only $13 for a 4x8 sheet at the local home improvement store and I'm sure will still stand up to some use. It's cheap enough to replace if you have to, and still looks good.
They had some actual plastic sheets there they promoted as ideal for bathrooms. That's actually pretty close to the picture above, just not as glossy. However, the going price for 4x8 was $34. With a 7x12 you'd need atleast 4 of those...