1. Your 134A level may be low and the line from the engine compressor to your bedroom AC is shorter than to the dash AC?
2. You have a separate dryer for each of the front and rear AC evaporators. The one for the dash is likely on the curb side of the engine. The other may be in the bedroom overhead cabinets. The dryer for the Dash AC may be plugged up?
3. You may have a leak at the evaporator at the dash or it may be plugged up?
If #1 is the cause, eventually the rear AC will blow warm also.
Cheapest fix is to borrow AC gauges from Auto Zone for free and install some additional 134A as needed. Cost about $10 for 12 oz. Then see how long the improvement lasts.
Your system probably has 5lbs or so of 134a when filled.
The oil and 134a specs for my AC are on a label on my Cummins surge tank.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs