I would check with the building department and see if the wall is required to be fire rated. If it is a separate structure from the house it likely does not need to be FR. But a check to them will settle the issue. Or perhaps your contractor knows.Sent 40 years as a structural engineer doing building structures. One of our engineers went into sales for LP and he showed our company their new product.
LP (louisiana pacific) has an OSB panel that is fire rated called Flame Block.
if it has to be fire rated you could go with Flame Block, sheet rock by itself, or OSB over sheet rock.
If not required to be FR I would go with OSB or plywood painted a bright white.
Good luck with your project. I am envious of all who can store their RV at home.
http://lpcorp.com/products/panels/lp-flameblock-fire-rated-osb-sheathing/