I would be worried about it with a big load like a RTT. That is a lot of weight up there. Over time, fatigue will do what it does best.
I have looked at a few of the racks, and decided against all of them for a RTT application for my own jeep. I tend to push my jeep a bit more than others...so I also tend to go for more robust designs. Big loads mounted up high put a lot of stress on the mounting points. Add in hundreds, even thousands of cummulative miles of rough roads at moderate speeds...I would worry about anything supporting a load which is mounted to sheet metal.
What is on the *inside* of that mount? How much surface area is the load spread out over? If the load is spread out over 9 or 10 square inches per attachment point, it probably will not be a problem...but if you are relying on 2 or 3 square inches from a thin, stamped fender washer, I'd be looking to beef it up.