On a 343RL you will need an aluminum stud finder; https://www.zircon.com/categories/metal-detectors/
Don't expect to find much in the way of studs as the side and slide walls construction is mostly a sandwich - 1/8" Filon outer skin/1" foam/ 5/32" interior wood. An awning rail will have screw holes about every 4 inches. By installing right at and under the drip rail, you will find some support that would normally be used for a topper. This may work . With the existing awning, there also is no place to bolt the forward arms between the door and main slide.
If you are considering adding that awning to the slide - there is no support structure to hold the awning rail
Adding a usual Dometic or Carefree awning above the 13 foot long main slide is kind of a waste as you really wont get much help shading anything as that slide goes out about 3 feet, leaving only - maybe - 3 feet effective.