As you can see in the photo, the outside skin (the fiberglass) and the plywood behind it is only about 3/16 inch thick. Behind that is Styrofoam.
When I attached the shower box, I originally wanted to drill holes for bolts to go through the entire wall and put nuts on the inside of the wall. For me, it would have required about a 3 inch, very thin bolt and I needed 10 of them, and I did not want to make another run to the hardware store. I have a lot of odd-and-ends and a mountain of machine screws, wood screws, and dry wall screws. I figure I had something I could use.
I finally decided againt going all the way through the wall and I ended up using self cutting sheet metal screws. With the Butyl putty completely surrounding the box, the stickiness of the putty itself holds pretty good. So, I felt comfortable using the self-cutting screws... especially since there were 10 of them.
I have a rear camera on my camper I installed myself. I removed the original fake cover and it was held on with 2 small screws through that same skin material. When I put the real camera on there, I used a slightly larger screw (diameter) so it would cut a new thread in the skin and a LOT of Butyl putty under it. It's held for 4 years with no problem.
Here's the screws I used, #10 one inch self drilling.
To plant a garden is to believe in tomorrow.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually