Is it leaking at the glass or around the frame? If it's at the frame you can probably just use silicone sealant around the edge. If you have tried that and it didn't work it may not be the window that is leaking, just showing there. If the window is on the "low" side of the A/C try tightening the bolts in it. Also check the roof and any marker lights close by. Also check the awning bolts if nearby.
Sometimes you run into a situation where you have to puu a window out and replace the putty behind the frame. All you do is remove the window, scrape the old putty off the frame and the coach and put new putty around the hole and install the window. sometimes it is easier to put the putty on the frame of the window. either way is OK.
The only way to seal a leaking window so that it will stay repaired and not begin to leak again in the future is to remove it and replace the butyl tape that is used to seal windows and other openings. If desired, caulk can be used in a crown layer over the top and sides of the window, but caulking will only be temporary, if it works at all as a fix for window leaks.