I would suspect tour leak is actually from the vent on the outside of the trailer which is just above the point you mention. The vent caps used then are notorious for it. You will need to remove the vent cap, replace the seal around the vent pipe, then reinstall the cap and seal around it as well.
Craig and Carol
1997 34' Excella 1000
1970 27' Overlander, International
2009 Ford F150 5.4L
ProPride hitch with 1400# bars
Past President Southeastern Camping Unit (12)