When we were gutting Peanut, we came to that really NICE ceiling and could see it had a great deal to do with the structural integrity--and provided built-in guides for replacing things inside as well as being somewhat integral to the upper cabinets, which we were lucky enough to have intact (except for the delaminated cabinet doors which Paul fixed).
When we took out the "putin" niche ceiling fan (disintegrated and leaking) we were able to peer a little between the ceiling layers and could see there was already some insulation in there. It was attached very tightly.
We ended up leaving the ceiling completely alone except for patching the putin ceiling/roof and painting
only the putin ceiling. Luckily the POs had all left it alone, too. As it was, once the supporting walls/galley supports were out, we had to put in a temporary brace, but that was easy. (Pic below)
Luckily, too, before we really tore things up, we spent a lot of time looking at FGRVs many threads and YouTube and online...I'd like to pat myself on the back for leaving the ceiling alone, but it was partly just the luck of finding the right threads and sites and having some good timing--not going too fast, and being somewhat lazy, I could see it would be more work than it was worth to mess with it.
We kept the temporary plywood and 2x4 support inside and as untouched as possible until we had the closet walls, the putin wall, and the two galley supports in place and secured. THEN we finally took it down. As it was, we had eased the ceiling/roof upwards over several days, tap by tap, so we didn't overstress anything but got the ceiling/roof back into what seemed to be a natural original slight arch.
It kinda breaks my heart I didn't jump in immediately and caution against taking out the ceiling--I wish I had. It could have spared you a deal of work.
As Glenn said--how often have we heard, "I have gutted my trailer..."
It took Paul 9 months, 7 days a week, 10+ hours a day to gut and reassemble Peanut. We saved everything then to use as patterns, even if we ended up making a few small changes.
Good thing he was retired but still able to do the work. Not sure we'd want to take that on again--ever.
Having pulled the ceiling out, you'll have to figure out other ways to fix it. Plenty of people do put in added bracing, made from wood & fiberglass, metal strips, carved things designed specifically for that space...it can be done, and now you have no choice.
Good luck. Let us know how you're doing with it.