I would suggest a search on the forums for sealing all of your ceiling vents. I went to Lowes and picked up a large roll of Aluminum AC tape. We have a 35QSIK with 2 units. I went through each and every vent as well as the returns on both units. I found numerous locations where the vents were misaligned or just not sealed at all. I also sealed the returns and main AC outputs where they connect to the trunk lines. We can tell quite a bit of difference in just doing that. Before doing this we were experiencing the same issues that you're talking about... Hope that helps.
2013 GMC Denali-HD, 4X4,Diesel, Allison, Crew Cab, Dually, Demco 21K Dual Pivot 5th Wheel Hitch
2013 Sabre- 35QSIK-6 Platinum Edition, Fireplace,Wood floor, Central Vac, 13K secondary AC, Washer/Dryer Combo unit, 4 point auto-leveling, Trail-Air Suspension, Trail-Air 18K Air Ride-King Pin Box .