We decided to start our window installs with the barn door window and the RV door window. We will put the 2 side windows in next. The windows are 14" x 21". We bought them from Vintage Technologies. He has many different trim rings to fit most wall thicknesses. Our RV door was 1.5" thick and the rear barn door was 1" thick. We lost the rear door pics in the tiny pic disaster. Same window but different trim rings. The first thing we did was make a template out of 1/8 inch masonite. We had to make a couple of them to get it just right. We left an extra 1/8 inch of space vertically and horizontally in making the template.
Here is the template on the window when we were checking the fit.

We next taped where the window would go so we could use our dewalt saber saw with a fine metal blade to cut the opening for the window. When using the saber saw it is important to have a long enough blade, the correct blade for your material, cut slowly and hold the saw flat against the surface so the interior and exterior hole will be in alignment and the same size. If you do not put the tape on the door or your saber saw you will scratch up the finish on the exterior of the door. We measured and put pencil lines for the top, bottom and sides of the window. We measured from the top, and side of the door as a reference point for our measurements. We then held the template in place to check our vertical and horizontal lines and used the template to trace the corner curve on the door.

THE rest of the pics were lost during the tinypics downfall. We then checked the fit of the window and the trim ring. If the inside is off a little by being to small, you can cut from the inside making sure you do NOT cut the outside any larger if it is the proper size. We had to enlarge the inside of our first window. We then screwed the trim ring to the exterior part of the window. We are going to have a wall 22" from the back of the trailer which will give us an area to put our water heater, battery chargers, propane bottle ect. We will mount a 5000 btu ac in the wall it will exhaust through the window we just installed. There will also be a window in this back wall. The passenger side barn door will have a maxair fan in it. This fan will pull air from the door window across the back of the trailer if the a/c makes it too hot with just the window. Also if we are not running the a/c we can open the window in the back wall, turn on the fan and it will pull air through the trailer when the door and windows are open to keep it cool. Here are the two windows installed.

We will make the A/C opening in the wall with removable blocks to fit a 8000 btu A/C if the 5000 btu A/C which we already own is not large enough. I think Don and others said they use a 8000 btu. Our trailer is 8.5 X 18 with the V-nose. We have 2 inches of insulation in the ceiling and 1.5 inches on the walls. Comments and suggestions appreciated.
Richard