Hmmm, that's sitting directly over the frame. The only stress on the aluminum is compressing under the weight, and resisting the toppling forces. I bet you could have put that camper on without the tray. Not that I would have but probably could have been done. I'd be more worried about the bed attached to the tray than the tray failing. You may want to attach the camper to the truck frame (drilling thru the tray) and then you won't have any concerns.
One more thing, you should make certain it takes some special tools to separate the camper from the truck. I'm reminded of my Yakima bike rack: it has locks to keep the bike attached to the rack and keep the cross bars attached to the car, but the rack to bar interface is 3 large wingnuts. Guy up the block had his bikes stolen with the rack attached.