If you are looking for the classic beauty of a woodie style camper, you can achieve it.
First -- 10' long plywood is available. My nephew got me some that was 4'x10' so I did NOT need a seam in the wall anywhere. That makes things a lot stronger and cleaner looking. It's not cheap mind you. I paid almost $100 PER SHEET for the stuff (with delivery). But I think it's worth it. That is my exterior sheet of wood. One side is grade 'A' stainable. You may have to go to a small-town, old-school lumber company to order it. The big-box board stores might not know how to special-order it.
For the finishing coat, I used a product called CPES. Stands for Clear Penetrating Epoxy Sealer. Do a Google search and you'll find it. Again, it's not cheap. But everything I read said it was worth it. It is a thin (like water) two-part epoxy coating. You stain your wood as you normally would. After it's thoroughly dry, you apply the CPES with a brush or roller.
You can see more on my build log here:
http://webpages.charter.net/outtahand/littletow/index.htm