Not only does it help preserve the paint & decals and make it look nice, it will help get rid of black streaks. If you haven't had an unwaxed RV over a rainy/wet winter yet, you won't know how annoying black streaks are, especially if you have aluminum siding...
It may seem like a lot of work, but it'll be easier than trying to get those stubborn black streaks off. Helps if you pick the right wax - ours says "easy to use" on the label. :) Also helps to have a fiberglass exterior.
Need to be very careful if you use a power washer. Not recommended because you can force water into cracks and places where it shouldn't be.