There's a pretty good discussion on mounting RTTs in
this thread. Check out post #144 from Mike S, who owns/operates AutoHome. He runs a Columbus on an unreinforced ARE topper and says that mounting RTTs to unreinforced tops is pretty common.
I've got an ARE CX shell and reinforced it. Check out posts #4 and #8 on
this thread for more details. I got my shell before I knew I was going to put a RTT on it. Once I got the RTT, though, I knew I was going to be living out of my truck for a while and was worried about the stress of my and my girlfriend's weight (way more mine than hers) up there every single night. Probably wouldn't have been an issue, but for the fairly low cost (~$300) of the reinforcing frame, I was happy to have that peace of mind. If I were only using it for occasional camping trips, I probably wouldn't have bothered reinforcing it.
Having said all of that, if you know that you want to put a RTT up there and haven't gotten a shell yet, I would recommend getting one of the burlier toppers that beavis0076 mentions. You get the peace of mind without having to deal with putting an internal frame in.