help finding stud frame

There are many ways to find a stud. Since you have an aluminum sided trailer, it is even easier than fiberglass. The easiest way is just to sound it out from the inside of your trailer. You will get a hollow sound where there is no stud, and a more solid sound overr the stud. You can also use s studfinder on the inside. Some work on density, others are metal detectors. Either would probably work. When you find the stud, just measure from some object (the window, etc) and transfer that measurement to the outside. Or pull the cover off whatever you have vented in your photo. As someone else said, there will probably be a stud on either side. You might even be able to see it wirh the vent cover off. Depending on where your floor is, there will be studs to the level of the floor, but probably not below. It is really not hard to find the studs, I've done it to drill holes through the side of the trailer and through the studs and into the interior of the trailer to mount a porta bote, and I didn't miss once.