Yellowish spots on the white ceiling

[dpam]

While sitting in the monty I looked up and started to notice a few yellowish spots on the ceiling. There were a few in a row about 3 feet from the back wall and a few on the ceiling in the middle of the coach. I initially thought these were a rsult of a roof leak, so I checked the roof and the rubber roof looks fine. I'm wondering if these yellowish stains might have been on the white ceiling before it was installed. Does anyone else have any of these yellowish stains on the ceiling? The stains range in size bewteen a 50 cent piece and a silver dollar.