We are in the process of doing the same thing and are still undecided about what to put on the ceiling. I was looking last night online and came across this product.
Ceiling Tiles by Armstrong
I'm not sure it this is feasible or not, but looks interesting. I don't know how flexible it would be to do the curved part of the ceiling. We have also toyed with the idea of doing wood, but I don't want to feel like I'm in a coffin!
The other thing we are doing is putting styrofoam (sheets cut to fit) insulation on the ceiling. We thought that if we had a small leak later on, it wouldn't ruin the insulation like it would fiberglass batting.
Would love to see some interior photos of your Argosy. I keep looking to compare. Ours can be seen in my gallery.