Take the AC out of the window... Put a new window in and seal it.
Now if you want AC there are much better locations you can put the AC. I have videos on installing an AC unit in your trailer and there are lots of photos and examples on this forum of where to install an AC and how it was done.
The weight of an AC unit combined with the trailer bumping down the road will never allow you to seal it so it will stay sealed. Besides, it really looks tacky. You can hide the AC unit inside a cabinet, over an existing cabinet or at the top of one and vent it properly so it works right. You can also drain the condensation just like you drain the sink. Some people put them on drawer slides so it slides out the side of a trailer while in use and then slides back in when rolling. All you see outside the trailer when the AC is not in use is a vented door.
Go take a look in this forum. You can also take a look at the AC I installed in a Deville. We've done this to all sorts of trailers including Shastas.
Post a few pictures so we have a better idea what the PO did and then we might be of more help. There are many different was to install A/C and sticking one through a window will work but keeping them sealed is the tough part.