I think you probably already know my input regarding 50 amp & second A/C from my earlier comment to you. Obviously a primary consideration is where you will use your coach--if you just never get into REALLY HOT weather, you may never be uncomfortable with the single A/C. In southeast Oklahoma last year we had some multi-day strings of 108-112 degree days (not uncommon for us). Unlike the desert heat, humidity was also high adding to the heat load. One can run into similar conditions traveling in other areas of the south and gulf coast. There is simply no way a single 13.5 or even 15K BTU A/C can maintain reasonable conditions in a 28-foot coach under those conditions (insulated windows or not). If you can avoid those circumstances, a single A/C may be all you need, but chances are that over your ownership some of your travels will result in that type exposure. The other obvious consideration is that the 50-amp service and second A/C is one of those things that needs to happen in the initial build, if you ever want it--adding later would probably not be impossible, but would be a pretty major project.
I first became aware of information on the Houghton A/C imported and sold by Advanced RV on this forum. Initially they were not making them available for direct sales to consumers, and by the time they started that program it was unfortunately too late for me to explore the possibility of coordinating with Phoenix to see if they would install them in my build. I don't have experience yet with living with my Dometic units, but the single most frequent complaint by far deals with the noise. Since I took delivery in February, I only briefly started mine to ensure they're working properly--but they are loud. If the Houghton units reduce that noise as much as all the information implies--I believe I would have invested the extra money to install them if Phoenix is willing.
All the best, Mike