A source for the skylight; J. Freeman, Inc. (617) 282-1150 www.jfreeman.com/skylights.html
They have a Curb-Mounted Double Dome (need curb mount not flat flange), model CD16X24. This fits a Curb OD of 17.5" X 25.5" and there is a tolerance of 1/2" it fit my measurement of 18" X 26". The cost is only $57.68 plus shipping.
Someone posted that the National skylight was MFG by www.foxlite.com so I called them to confirm. They still make the double pane rectangle skylight that everyone has on their National RV. Part number is CDE1624 the cost is $69.90