Thickness of side wall near shore water supply inlet

Click on your link and click edit. "Thickness of sidewalk near shore water supply hose" I think you meant to say SIDEWALL. :).....Dennis

Use Olympic rivets, aka tri lobed or tri fold. They expand with three ears, and are used to mount hardware to fiberglass body. If you are going through the whole side, use a machine screw or bolt and washer.

Great idea with paper clip! Thanks

All the way thru the wall? 1.5 inches
Thru the skin 1/8 to 1/4 inch plus washers rung etc..

Title says "sidewalk" not side wall

You are probably going to use 1/4" pop rivets so I would tape the ring in the location that you want and then drill one, or all four holes, in both the ring and the sidewall. After you have drilled the holes, you can insert a straightened out paper clip, or other stiff wire, with a little bend on the end. Insert the wire and hook it on the inside lip and use your fingers to hold it at the edge of the ring. Pull it out and you have your wall thickness. Then head to Home Depot, Lowes, etc and get the right aluminum pop rivets.
Be sure to check your location so that you are not drilling in to any frame members.