Originally Posted by Cpt. Ron
Frank is right, that was a problem in the making quite a while ago. I had a similar crack with another gusset further back on the submframe...
Airhead subframes have been breaking since the '70s. The solutions are pretty obvious: brace'em, with a diagonal or a trellis. That way, you could move the failure point forward to the attachments.
Or carry less weight back there.