Some are suggesting to add the second unit I did not want to have to run the wires and such plus my rig is a 30 amp power supply. And as for the wiring on domitic site it says the 13.5 and 15 BTU unit use the same size wire. And is there a big difference in brands? Is one brand better than the other?
Like others, I don't believe you will see any difference.
We camped many years in the beach-side campground at Fort Clinch and Gamble Rogers state parks. If you have visited either, you know there is no shade. Our 15,000 BTU unit would keep the trailer in the 80's when outside temp was 95+. I broke down and installed a second 13,500 unit in the bedroom with it's own 20 amp breaker box and wiring system. We can still use 30 amp main service and use the additional 20 amp outlet for the front air.