How we attach the strut to the side of the trailer really depends on the trailer.
The top trim piece that comes with the trailer is screwed to the top of the wall steel members using a #14 x 1-1/4" hex head usually but they vary on where it is on the trim. Sometimes it is low and sometimes centered top to bottom, this will determine if you need a spacer. We reuse the holes already in the trim if centered and no spacer needed.
If a spacer is needed to miss screw heads we use a piece of 1/8" thick x 1" wide aluminum flat stock.
Use 1-1/2" #14 self tapping hex head screws to attach the strut with 1/4" x 1-1/4" fender washers.
We use butyl tape if we need the aluminum spacer (it goes between spacer and trailer so it fully seals holes) but if no spacer needed to keep strut flat then we use a quality silicone under the fender washer so it is compressed when tightened, we apply the silicone BEFORE inserting hex head, pre-drill if needed. This way silicone is inside the hole and under the fender washer. When tightened it should fill the oval hole in the strut and the fender washer should cover it and seal the penetration.
The beauty of the strut system is the ability to add additional features, such as an awning, or privacy shower enclosure as shown in the pictures below. These can be added anywhere on the strut system to meet your needs.