My trailer only has 1/8" luan for the sides. So anything I mount is either directly to the metal uprights or I will take a "one-by" and span several uprights...then I can mount stuff using wood screws to the one-by stock.
You can see I did that with the helmet/gear racks:
View attachment 176206
Omar is right on. Mine are attached to the metal studs with hex head screws about 1/2" long so they will not penatrate the aluminum siding. My units are NOT predrilled for mounting as there are many different stud spacings. For the most part, the caddys will hold alot of weight. How much weight you can put in them will depend on the strength of the wall you hang them on. I do recommend however, if your using the single caddy for the 5 gal. plastic jug, that it is installed on the trailer floor.