this is a picture from the Jayco website of the slx frame.
As you can see the frame member going out to the side of the trialer is beveled so it was easier to attach to the main frame. Not better, easier and of course and most important, CHEAPER.
Mounting them on the main frame puts them nearer the center of the trailer and thus a bit less stable.
Maybe weld on a support bracket to the beveled part would be the answer. Hey, while you are there, have put on a receiver on the back for bicycles or stuff.