I have used two products on water leaks with good success.  First, you could try Flex Seal (liquid).  This would be good if there is some flexing of the connection occurring and because you can easily work with it and layer it.  It is possible that the crack is due to flexing of the trailer going over bumpy roads rather than a collision.  Second, you could try Water Weld ("plumber's putty").  The advantage of this is that the area does not have to be dry to work.  The disadvantage is it can be difficult to work with because it is an epoxy and you don't have a large window of time after mixing before applying.  It is not flexible.  You should try to rough up the area with sandpaper before applying either.  Both can be purchased at Home Depot, Lowes, Menards, or some hardware stores.