I used rivet nuts extensively in my job and they will hold as well or much better than a self tapper. Are you telling me there is no part of the bracket that goes over the trailer frame? I find that hard to believe since there are close to a thousand lbs. of downward force on that bracket and even more when the tow vehicle hits a dip in the road. That single self tapping screw is NOT going to hold anywhere near that kind of force. A structural part would have a bare minimum of three but more likely four grade five bolts. The best way to go here is probably the phone call.
And referencing this link http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=517:reese-dual-cam-setup&catid=26&Itemid=132, the brackets most certainly do go over the frame rail.