That's a bit of a tough one. Do you have the whisper quiet unit in the main living area or the center intake with dump switch which would be the Brisk II unit?
I have a 2017 371REBH which is a very similar floor plan. When the driver side wall is facing West in warm weather, the bunk room does warm up pretty good. However, I think there might be more challenge in the Loft design. You'll likely need to reduce airflow in a couple vents. For starters, reduce airflow in the bathroom by about 80% and about 60% on the driver side vent in the main bedroom. If there's a vent over the loft, then reduce 75% on that one as well. This may help increase airflow to the vents in the bunk room. Since mine was aligned with the bunk room on the East for a while, the bunk room was actually cooler and I had to reduce air flow to the bunk room vents to get more airflow in the living area. I popped the caps off the air ducts and used strips of painters tape.
If your unit came with the whisper quiet, then that unit may not be moving as much air as it was designed to move. I had to have mine modified with an additional intake vent to get better air flow and performance from it.
Hope this helps.