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I looked up the floor plan you where describing. Sunline Coach Owner's Club - Sunline Owner's Files - 1994 Sunline TT, TC, & FW Brochure
When you said sky light I was thinking a roof dome but then looking at the floor plan I believe you are talking about what we call a roof vent. It has a crank up cover. Looks like this on the inside
And like this on the outside
Your vintage may be a little different but is this what you are referring to as the skylight?
You asked about if they made these out of mold resistant materials, well no. Having dealt with water damage myself and knowing what it can do, I would be cautious. If you are seeing wrinkling ceiling that is for sure a heads up.
If it was from a real long time ago and the leak repaired and the camper dried out, then the damage may not be as bad. However... get up close to the ceiling where you think the leak is and sniff. If it smells damp and musty odds are good the wet is still in there.
I do not know how willing the owner will be but that vent shroud does come down pretty easy. Again your vintage may be different but if you take the plastic shroud off it exposes some of the wooden opening that you can then look at.
If you are very handy with carpentry you yourself can fix this regardless of how bad it is, but.... it will cost money for the materials. You may have something mild to full blown roof rot and wall rot pending how far the wet went.
If you really like the camper, the rest of it is in good shape, you are handy with wood and you can get the camper real cheap and I mean cheap, then you could be buying yourself a summer long fix it project and then have a real nice camper once all done.
Water leaks are about the worst thing to look out for as hidden damage can be real until it is checked out.
I do not want to get you over concerned but need to know going in eyes wide open what to expect. We have had several here on the forum do major rebuilds. It is all doable and can be fun if you are prepared to do the work your self. Hiring it out pending how bad it is can add up quick and real bad can be in 4 digit numbers.
You really do not know until you inspect and often times until you drop the ceiling if you see water signs know how bad it is.
If you can get a ladder and look at the roof look for large cracks in the caulking that allow water to get in. Look at all the seams and around any roof penetration. Do not walk on the roof. Most are not walk on roofs with plywood under them. To service the roof you lay down a tarp and put plywood to span the rafters. And in the case of water damage I would check what you can before attempting to go on the roof even with a tarp and plywood until you know the rafters are sound.
If you do decide to get it, we have many here ready to help point how to do repairs and even some how the inside is made.
Good luck and hope this helps