Unless your ceiling is significantly damaged and you plan to replace it in some way, my suggestion would be to leave the ceiling in place and to search for the leak in some other way. I'm thinking removing the ceiling without a lot of damage to it is impossible. Also, even if you open up the ceiling in some area, it will probably not be helpful because of how far a leak can travel before it becomes apparent.
Leaks can be found with a pressure test. Dealers with the required equipment can pressurize (slightly) the interior of your trailer with a powerful fan and do a soap bubble test to find the leaks.
You've searched the roof carefully for leaks; how about the clearance lights? Air conditioner roof gasket? Antenna "O" rings? And any fixture on the side of the trailer.
I think if you keep looking for the leak, you'll find it.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch