The inside of our reefer trailers are covered with what appears to be bathroom wall board. The sort you see in public restooms, sort of a pebble finish. Simple to clean unlike wood, reflects light unlike wood. It is backed up with insulation in our trailers, roughly like blue board. I would do this if I were going to finish one off.
In addition I would install E-Tracking down both sides. Serves as a tough rub rail and provides a wonderful tie down for securing loads. Get a couple shoring bars and a few straps and you will be the envy of your friends. Properly installed you can build logistical flooring to seperate filthy pipe from new tub surrounds for example. Or my idea bunk space for a simple camper for overnight hunting trips. I'll bet Grainger, or Northern Hydraulic has the stuff.