If you have a Lowes or Home Depot around, go to the section that has wall paneling. There sould be a few different kinds, The one that I am refering to for bathrooms and such. Places where you need something for a water barrier. There should be a couple of different styles. Some look like tiles, some just textured. They come in 4x8 ft. sheets. 2 will do the walls on the inside. * foot long and 4 foot up your wall. They also sell half sheets, so you can pick one of them up for the front.
You could use "Liquid Nail" to glue them in place.
I would also do something to the floor. You said you are going to waterproof it.
I would, just me though, go out and buy some 12"x12" black and white tiles, the vinyl type, where you peel the back off and stick it, Kinda like a giant sticker. I would do a checkered floor. It would add to the protection of the floor. the tiles are also pretty cheap.
If you did that, you would have a kick a$$ trailer inside, while making it look lighter inside so you can see better.
But that is just me...LOL