ALL National Products and ALL RV's have the same size hole for a RV roof top AC unit (14 inch square). Your AC units are made by Dometic. National always used Dometic. 1999 means you have the Dometic wall CCC 4 button tstat control system that uses a telephone type cable top communicate between the 2 roof AC units and the wall tstat. That system was obsolete in 2001. That means the easiest and simplest thing to do is purchase new Dometic AC units and I would get the Penguin Model, because THAT model has the new AC tstat control boards already installed. You only have to purchase the new 12 button Dometic wall tstat. The 1 tstat controls BOTH AC units. Just like your present Wall tstat. Doug