Just got done installing my door window after looking at it for a few weeks and trying to get up the guts to cut the hole, and thought I would share a few pointers. 

Take the door off the camper, while it has multiple hinges and a lot of screws, it is very light and easy to handle. The friction hinges will not be upset if the door is removed.

Have a nice flat surface to work on, this makes it easier to lay out and set the window. 

Use painters tape to prevent the shoe on your saw from scratching. I taped both sides as I cut from the inside and thought it may prevent bad things from happening on the outside.

Speaking of the outside, if I did this again that is where I would make my cut. I figured the saw blade pulling up against the door would be easier on the fiberglass panel, than the cut pulling it away from the door. I used a small saws-all on the long cuts, but since the blade was fat I used a jig saw on the corners. For some reason, and despite me holding the saw dead level, the blade wandered outwards when I cut the corners. This left the inside ring fitting perfectly, but the outside corners which prevent water entry too large for the gasket to contact. I would much rather have the inside too large! I dug around and found a tube of caulk, naturally it was white instead of clear, and generously caulked the corners, and ran a bead around the sides as well. This seemed to work well but left a lot of cleanup! 

Others have had problems with the mounting screws, I actually found them easy to work with. They are self drilling screws, just be careful to line up the inner ring with the channel on the window they are designed to go into. Use a power drill with a clutch so you don't overtighten them. They went in fairly easily and I hand tightened them to prevent stripping.

If it hadn't been for the wandering blade in the corners, this would have been a fairly easy project and the results are worth it! Maybe someone has a tip to prevent this, I am thinking a router may have been a better tool to use.