Yep, its a moisture problem and had the same thing happen to my TT.
As mentioned, check entire roof, roof/sidewall joint, marker lights, windows, vents, etc.. If the FW has the Filon fiberglass outside walls, check for delamination...., it looks like this:
Bob
__________________
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)