Roy the rivet vs nuts/bolts debate has been going on a long time around the FBT website. I personally think pop rivets are not as good as using screws with self locking nuts and washers, so I think your on the right track. I have seen rivets stretch and loosen up and this is what is causing leaks
that people have reported on this site. I have been using stainless #10-24 crosstip panhead screws with the matching self locking nuts for any new supports and to replace a few loose rivets I have found on my 05 Scamp
. I also use washers under the nuts to distribute the pressure. A bonus for using this type and size of screw is that the head fits within the beauty cap retainer ring so you can put the little button caps back on for a factory look. You can also get acorn nuts for a nicer appearance on the inside of your trailer. I don't think you need a rubber gasket under the fastener head on the outside. Just put a generous amount of sealant in the hole and under the fastener before putting it in. So far this setup has not leaked for me.