I would consider drilling out the pop rivets and removing the whole unit. Clean all the old sealant off and reinstall using Polybutyl tape 1/8"x3/4". Much as I hate to suggest it, you could use pop rivets again, but make little balls from the left over tape and press firmly into the centerhole of the pop rivet to seal it. My preferred method would to be use the Stainless Steel Truss head bolts as discussed in this thread. If mine was leaking that is how I would fix it. There is a moulding, or one could be added on the inside to hide the nuts or rivets.
Adding any sealant around the flange is not really going to solve your problem, only temporary. Remember these trailers are in the 30 year old range and the stuff they used then has stood pretty good for this long. But its time to fix it. A lot of the products have greatly improved over the years, which is good for us.
Hope this helps.