From what I have read, they are about 600 pounds empty. I have thought about doing the same thing with the RTT. It seems like most people that do one on them, they just make rails, but they are right above the tub, in which case you would lose the functionality of the changing rooms that the RTT's have. I have seen someone else build a bed rack type system that puts it up to the height it would be at if it was on top of your truck, but that does seem like that would be top heavy that high up. The best of both worlds would be this:
But you can't buy one and you would have to fab up yourself or have someone else do it... I would love to build one like this, but I don't have the equipment needed to do it.