about 8 years ago, I purchased windows to match my wilderness rig. Somewhere on the frame was the manufacturer's name (Kinro, Inc).
I contacted them, found a local dealer and was able to get two windows from them. They were small - 16" wide by 21 Inches tall.
I had the issue that Drew pointed out. you need to see if where you want the window placed is crossing a support beam. If you do, you have to box it out like you do a stick house (post and beam construction), but that can make a mess out of your side wall(s).
If you don't have anything in the wall where you want to place the window, they are compression fit (screws on the inside, butyl caulk and caulking on the outside).
I'd use a stud finder to see if there are wires or studs in the walls where you're cutting....