It can & does happen more often than we as individuals see, hence the recent problem at Zion, and the I-40 slide,that Homer referenced, but most are much smaller, but a basketball sized rock coming off the side of a mountain will put a hell of a hole in an aluminum or fiberglass skinned RV.
There is really not much you can do, except keep your eyes open, and be aware of the possibility. I have seen Many rocks that have fallen, luckily, they were all on the shoulder.
I agree with others in saying that you have a far better chance of hitting something off another vehicle, like the semi wheel, (tire & Rim) I hit with a company van near Pittsburgh, or the many battery battery box covers i've seen laying in the road, or the entire fiberglass (or plastic} front bumper cover laying in the fast lane on I-95. Just be careful.And let's not forget the multitude of Goodyear Gators out there.