I live in AZ, and my FW is in the sun every day. One of the first things you'll notice is that exterior plastic parts start to show damage first. Usually in a year or two. Items such as vent covers, Stove vent flaps, etc. will get brittle and crack. I eliminated this by painting them with Krylon spray paint for plastic in gloss white. Once I did this, I didn't see any more sun damage.
After the plastic parts, the decals on the body don't do well either. They have a tendency to get brittle and peel on the edges. If I see one starting to peel, I take a razor blade and trim it very carefully. In most cases, you can't even see this.
Next to experience problems were the tires. I put on 303 every month or two, and keep them covered. Other than my spare which was exposed to a southern exposure every day, my tires have fared well.
I tried covering the whole unit with one of those after market covers, but I gave up on it. It was difficult to put on and remove, looked bad, and made it difficult to leave on a spur of the moment trip.
As for the inside of the rig, we've never really had any heat related issues at all. By and large, the unit is holding up very well for six years sitting in the sun.