Oh boy... The good news is in your other post you stated you are handy fixing things around the house. If you are handy with wood and have the tools they will come to use with this project.
The issue is above that wet ceiling board is fiberglass insulation that is most likely soaking wet. And by the looks of the pics that are really deteriorated, it has been that way for over a year or more.
If you are looking for a recommendation, after dealing with my rotted slide floor, the ceiling panels that are wet need to come down to inspect above. I myself would start with the one that is deteriorated first. It has to come out anyway. Once out you can may look to see how good or bad the rest of the situation is.
I do not know how the older Sunline ceilings where held up. On the new ones they are glued and stapled and have a plastic seam channel that hold the 4 x 8 sheets at the joint. Yours is a variation of this. A good sharp sheet rock knife may be a place to start to trim out a piece and then keep going.
You have to get all that wet insulation out of there. Dry it out 100% and then if the rafters are still hard and intact they may need treatment from the mold rot to kill the fungus so you do not get dry rot later one. Here is one such product Wood preservation, rot repair, and restoration using epoxy resin on boats and homes.
Looks like this
And a heads up, buy the proper rated respirator when using it. They sell one that is a reasonable price for what you are getting. The fumes are "bad" but with the respirator on, you do not inhale any.
Now what to fix it with, If you are anywhere near Elkhart IN they have many surplus places that sell camper everything. If you need some names let me know I'll dig up the ones I have been to. If not then you might find a match in a local lumber yard of paneling that will come close.
Hope this helps and good luck.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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