Follow up: Draw the lines for the window cut out on the masking tape on the outside from measurements made from the exterior top trim. Just inside the top, bottom and side lines, drill 8 holes with a 1/8" or smaller drill bit. This will give you two points on each of the four sides to draw the line
where the flat meets the radius corners.
Don't drill holes on four square corners outside the radius corners. My favorite alternative to this is using a long, metal-cutting, Bosch blade in a jigsaw. It cuts all the way through the skin, a rib and the interior insulation and panelling in one pass. The window and the trim ring fit perfectly. This only works IF you, a) are not cutting through more than one vertical rib and don't have to reinforce it with steel cross members, or b) are installing the 12" wide vertical slider RV windows that fit perfectly in between ribs. With this method, you can wire, insulate and panel the interior walls first.
Best wishes for a successful build.