I had the same problem with the dash air in our '04 Gulf Stream. Last June while at the Gulf Streamers Back Home Rally in Indiana several of the suppliers were there including Evans,the manufacturer of the dash AC unit.
I told them of the problem I was having of no cool air. They had the tech come right to my coach and he found the problems right away.
The heater control valve located in the heater hose line right beside the air unit in front was not closing all the way.
BUT, the main problem was the condenser fan was not running, which it should be anytime the air is on. This fan is located between the frame rails in front of the front axle on our coach.
The tech went and had the Gulf Stream techs that were at the rally come to our coach right away and they found that there was a problem with the fuse block and no power was getting to the section of the block where the fan fuse is located. They repaired this, the fan came on and now our dash air is COLD, very cold.
Don't remember the exact figure but the air comes out around 40 degrees on a 90 degree day!
I am happy and cool.
BTW, I forgot to mention that I had previously complained to our dealer about the dash air and they said they checked it and it was "okay".