Thickness of R9 studs on a 2017 starcraft 21fbs TT

Insulation thickness for a given R value will vary depending on the quality of the product, so it's tough to get the right answer unless you tear a wall apart to have a look. I'm sure you don't want to do that.

If you can physically measure the wall thickness, that would be the way to go IMO.

Really though, IF the studs are made from 20 gauge aluminum (maybe 1/16" thick), the wall covering over the stud is ¼" thick, your mounting bracket is ¼" thick, and you want say ½" penetration in the stud, then you could go with a 1 ¼" or 1 ½" stainless self tapping screw. Remember that the tapping end of the screw is maybe ?" to ¼" long and is not threaded, so has no holding power.

Sounds to me like 1 ¼" would work fine. Fill in the blanks with your variables if you want, but I suggest you get a wall thickness first and go from there. Hope this helps.

Looks like first link didn't work!metal studs

Stud thickness
These sizes refer to the width of the stud. The thickness of the metal can vary, with the most common being 25 gauge and 20 gauge.

Measure wall thickness at a window or door.

R designation is normally for insulation value and not structural.
Web says it has aluminum studs.