[xcntrk]

I have a similar model, front 2 AC's, and I too struggled with keeping the garage cool. Upon inspection I realized that both front AC's blow into a long air-duct that runs from the bedroom in the front - to the rear 2 vents the garage. There's one air-duct per side of the AC exhaust and all AC vents run off that central air duct. Granted the vents closer to the AC's see more air flow and tend to blow harder.

So what I did was switched out my AC vent bezels from the garage forward to a directional flapper unit that allows you to regulate the airflow coming out. By restricting the front vents you can push more air down the duct and to the garage area. To use rough guesstimate percentages; I now have 20% flow in the front bedroom, 10% flow in the bathroom, 30% flow in the living/kitchen area, and the rest pushes to the garage.

If there's ever a need for more immediate airflow up front (like when you first start up the unit on a hot day), you can adjust the vents individually, or open the large registers on the AC unit itself to blow air straight down. Lastly I have learned that pre-cooling the unit before arrival at your destination is welcoming too. I will fire up the generator and run the AC's as a first thing when I pull into a new campground. By the time you get to your site, setup, and your rig sees pedestal power, the inside will be nice and cool.