Heartland may be reluctant to give you much help on this as they don't generally get involved in customers making modifications; especially to the structure. You might be able to get them to send a drawing that shows the side wall construction - maybe. But you will still have to locate the vertical framing yourself. You shouldn't trust the drawing to give you more than a general idea.
Also, assuming an aluminum/foam/luan/fiberglass wall structure, you'd probably want to cut an opening between 2 existing verticals, to minimize structural impact. And then you'll have hanging vertical supports above and below that will have to be framed. And all this will have to match the size of a window, or the window will have to match the size of the cutout.
Some people have the skills to do a job like this and do it well. If you're in that small group, it'll probably be a nice modification to your trailer. If you're not sure about your skill level for this job, consider that once started, if you get stuck, you might find it very difficult to find help to get the job finished.
If possible, take a lot of pictures as you go. Someone might want to follow in your footsteps.