THERE IS NO WAY ANY RV ROOF AC WILL PUT OUT 28.8 DEGREE COLD AIR. PERIOD. The best it will do is around 39 to 40 degrees in lower ambient temps. Besides, IF it could put out lower than 40 degree temps the Freeze thermister would shut the compressor off. There seems to be a BIG problem with how you and this so called tech are using their Temp measurement devices. One thing that will cause a problem is IF the bad AC was replaced. AND they did not install the separating gasket. That would cause the cold air to be pulled back into the AC unit and cause a lack of air volume. It could have happened at Tiffin when it was built, but I have never seen the separator gasket missing from the OEM. Doug