To make sure it's the window and only the window use blue masking tape to tape split open garbage bag over window (cleans up easier than duct tape). Seal that plastic all the way around. If it rains and the plastic prevents your leak then you will know it is that window for sure.
If bag over whole window frame stops leak you can tape bag only to window and leave only source for leaks
the window to trailer wall that you just paid to have sealed.
Giving you better leverage to deal with installer, and if it's not the window frame to wall seal it could be a window seal like the glass to frame seal if your window has that. I could see water running down my screen from that seal being unglued at the top, your seal of window to window frame might be leaking along an edge and running down.
Are you missing any rivets or rivet covers where water could get in and flow down to window?
I feel for you, it can be so frustrating.