I mounted a custom cabinet I made above our toilet last year before we made a 7,000 mile trip on very rough roads to the east coast. It was secure and did not have to be tightened when we returned. I estimate it weighs about 25 lbs loaded with toilet paper and toiletries.
Here are pictures in my album:
http://www.keystonerv.org/forums/album.php?albumid=708
I used a stud finder and did find a narrow stud. I drilled very small holes that would be covered by the cabinet to better define the edges of the stud. Also as others have said, there is a "header" at the top of the wall just below the ceiling and a stud in the corner of the walls where they meet. I used expanding hollow wall anchors on the side next to the shower where I did not have a stud.
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