I have this exact same problem and this is the reason I signed up for this forum. I have a featherlight 25'. I called a local dealer and I called jayco and was told by both places the only way to fix it was replace the whole roof and it would cost 5k. Who builds a trailer with a roof that cant be fixed? are you serious?!?! Trailer roofs leak all the time and their plan is to have customers drop 5k. Unbelievable. Now that I'm done venting I'm wondering about alternative options. My parents RV. Has a thin Burbur "carpet " like material on the ceiling. I just want to make it look nice. If I remove the sagging wood on the inside and expose the styrofoam and get rid of all delaminated wood. Could I apply spray or roll on adhesive and apply this burbur ceiling material? Other ideas appreciated, it doesn't have to be stock, just structurally sound and look nice! And next time you build a trailed build it so the roof can be fixed.
Nice job on the repair. Now's the time to reinforce the roof for a party platform so you can re-purpose all the "weights" used to repair the roof. You're saving a ton of cash doing this yourself (obviously) and I'd definitely check out the roll on roof coating.
These forums are not only for the OP's advantage at the time of the issue but also for those seeking info in the future, finish what you start please!!