I would say that most use a higher grade of plywood (birch core) with a nice looking grain pattern. Others have used cedar strips just like they do for canoe building. The strips are typically only 1/4 inch thick and have a cope on one side and matching bead on the other so they go together nice and tight. If you want to go that route google cedar strip canoe building for some ideas and methods.
I used 3-inch wide cedar boards that are 1/4 inch thick with a ship=lap joint on the edges. I nailed them on with copper nails.

As for finishing, for plywood sides, some get by with spar varnish alone. The next step up is epoxy coated with a spar varnish finish on top for UV protection. If you use cedar strips or planks like I did you will probably want to use fiberglass and epoxy for the added strength then the spar varnish top coat for UV protection.
Like wooden boats, woody campers with a clear finish require some special care. If you can garage them when not in use the clear finish will stand up for a long time with little or no maintenance other than a good washing. Sun is the enemy of any clear finish. If the camper is stored outdoors you will have to sand and re-coat with spar varnish occasionally to maintain the finish. On wooden sail boats this is typically an annual event! I store my camper indoors when not in use (and its only a year old) so I haven't yet had to do any maintenance of the finish. Perhaps some other woody owners can share their experience.
Chuck