If the leak is around the window frame seal, then you need to buy some butyl tape and reseal the window frame to the outer shell. I used a white tape when I did my windows, but there is black too. Look on youtube for how-to videos.
Proflex RV is a product I see recommended all over (I have a tube in the parts box), but in pinch you could use a DAP brand sealant called Alex (acrylic latex). You can buy it at Home Depot or any other hardware store. It adheres really well to lots of different surfaces and is paintable. It comes in tubes for your gun, or in a pressurized, self dispensing metal can package. I keep a can and some extra Eternabond in the RV for emergencies.
Jose