It is impossible for us to tell you if the leak is in the air conditioner.
To my knowledge it is not common for a leak to occur at that point, but
anything is possible. Before you go spending a bundle on a new A/C unit and
drip pan (yes the pans are available only through Jackson Center or a
dealer) make absolutely certain that you really have removed all other
potential sources for leaks. Any and every seam and rivit are suspect as
well as any joints, like the roof vent neck, window frame, window gasket,
even exterior lights. You may need to look all the way down the side of the
trailer down past the top of the frame. If scrutinizing all these areas does
not help, enlist the help of a patient person. Give them a water hose and
have them VERY slowly work up the outside of the trailer giving the water
ample time to travel it's path before moving on the the next area. You will
be inside the trailer looking for leaks.
One time I discovered a leak coming in through the lower frame corner of a
window on my '60 Overlander which has similiar windows to yours. I never
thought of that area for a leak since it is not readily visible without the
window being open.
Hope this helps,
Scott