You did not specify what type of RV you have or are getting. Roof storage sounds like a viable plan, but you really need to know if your RV roof can even support the extra weight. It might hold the weight, but what happens when you go bouncing down the road and those "G" forces bouncing up and down on your roof. Most RV roofs are not made to support any extra load.
I'm not saying it's impossible, you just need to check out your RV first before doing this. One thing is certain, however you attach your supports, it needs to be spread out over as large an area as possible so the weight you add is distributed over a larger surface.
Another thing you REALLY need to consider. RV's are pretty tall. How are you going to get "up there"? Then, how are you going to hoist your items "up there" once you are "up there".
Before advancing further, I think it makes more sense to utilize the space UNDER the camper. Attach metal storage containers to the trailer frame, just make sure they don't hang down too low. It's much safer than crawling around on a roof, and the weight is lower.
Another thing about adding weight to the roof, this will throw the camper's center of gravity off. It won't take much weight on the roof to do this either. This will cause your camper to wabble side to side causing sway. Anytime you add weight higher up, the center of gravity raises causing a potential imbalance in the stability of the vehicle in motion.
Sorry, to bust your bubble, but you REALLY need to think these things through. It sound simple, but it's really not. There much more involved that simply installing a simple carrier on the roof. Much to think about.
I'm not saying it's impossible, I'm just suggesting you do your homework well before making this modification.