In my WA2916 if i close the front bedroom doors, and set the thermostat to say 68, that front room has to be in the high 70's. If I leave the door open, the temperature is mostly even and I am OK with it. I was thinking the same thing, that I need to find some of those brown circle vents where some of the louver's can be closed. The pressure should raise slightly in the rear and help.
The same goes for my white A/C ones, except the opposite in that the front room gets very little air, but the rear, ducts are much (by far) stronger flow. I should test this theory for the A/C by using tape over some of the vents I suppose
TH : 2016 Stealth WA2916
TV : 2012 Ford F-250 PSD