Just a small correction in terminology, "studs" are the vertical supports in your walls. "Joists" or "ribs" are the cross-members in your roof. I'm not sure what you'd call the horizontal members around the edge of your roof.
To the best of my knowledge, they are aluminum and you should be able to find them with a metal detector that will detect non-ferrous metals. Their locations are often also revealed in the morning dew as relatively dry stripes. There aren't as many of them as you might suspect. Your roof is a laminated foam/plywood "sandwich" that gets its strength primarily from it's laminated structure not its joists/ribs.
Here's a link to a graphic of the 23H's structure (see the last page). It doesn't have measurements but should give you an idea of their general location:
https://www.yumpu.com/en/document/re...ebago-view-23h
Personally, I wouldn't hang a TV mount from a joist/rib. Hanging it from one of the horizontal members around the perimeter of your roof would be much safer. Section A of the graphic shows the sidewall to roof structure.
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