humm, got a number of views but no one is answer up. So, ole motor mouth will try to help.
If the window has a clamping ring on the inside, remove all the screws and put the ring/screws in a safe place.
Using a thin putty knife (or wire garrote) cut through the sealant between the window and the fiberglass (on the outside). It's possible the window is really stuck. Don't yank on the window, it's aluminum and you'll tweek it. Pull window out. Clean off all the silicone and/or butyl or other goo someone may have used on the window/trailer. Now is the time to inspect the window and do any extra cleanup of the frame or window parts. To reinstall, use Butyl tape (it's a gray putty that comes on a paper-backed roll...cheap too). Put a layer all the way around the window, evenly. Put window in. From the inside, install clamp ring and screws. Cinch down screws going around the window a bit at a time. Go outside and using plastic knife cut the butyl that's been squeezed out. Wait a full day, cinch clamping ring screws a bit more...remove excess butyl.
For vents, etc. see info here: Butyl Tape, Silicone and sealing