Dont have a clue what to do. I have a double wide mobil home, bought it BRAND NEW in 2000. The roof is OSB with 30 year shingles on it. The ceilings are not flat, they run upward like the roof does.( not sure what you call that)

The ceilings are 1/4 inch sheetrock. I noticed in the spare bedroom a spot about 1 foot by 2 foot that was starting to sag so i reluctantly knocked a hole in it. Its all dry in there but we have not had much rain at all in 6 months, but i did see a small spot in the OSB right above the sag in the ceiling that looked like its had water on it so im assuming thats where the leak is, but the spot is not in a seam in the OSB, its right in the middle of it, no nail holes, seams or anything, so i dont know how water would just penetrate it and further more i dont see how its getting through the shingles, i got on the roof and it all looked just fine. I dont have the money to call a pro out here, and like i said, the home is only 7 years old and its got 30 year shingles and they look fine, what do i do? How do i find where the water is getting through on the outside of the house?