a/c works great, but doesnt blow hard.

The way I remember it is below the glove box. Remove the panel down by the floor.

 

The coil is probably very dirty and needs a good cleaning with coil cleaner.
To save hours of work to get to the coil I installed one of these, cleaned my coil and now run my ac on 1 or 2, never turn it to high.
http://www.cleanandcool.com/products/peterbilt-air-conditioner-coil-access-door-kit

 

On that model does it not have a fresh air vent in front of the right engine air filter housing? If the system is set on fresh air it is open directly to the core? Make sure drain tube is open. Wash it out with a water hose and blow it out with a long small diameter blow gun. Repeat back and forth intell it is all flush out the drain tube?

 

We are talking about the coils in the evaporator. Same as a heater core only for the A/C. You can get coil cleaner at the hardware store. It comes in a spray can. It is possible the squirrel cage vanes are full of dirt. I would check that out. I cleaned my evaporator with coil cleaner,a toothbrush and a shop vac. The best way would be to evac. the system and hot tank the evaporator. Mine came out fine as described above. Also in the future keep your fresh air vent closed. You will get less dust and dirt in the heater core and the evaporator.

 

I just took a hole saw and cut a hole. works just like that door, but didn't cost anything. Then I took some of that blue cut to fit furnace filter.

Cut a piece of filter about 2" by 15" rolled it up and stuck it in the hole. Those evaporator coils get caked solid with lint, dirt, pet hair etc.

Peterbilts should come from the factory with an access door. But then again Peterbilt could do a lot of things to make life easier owning a 1-2 million mile truck.

 

I used to know a lady like that.

 

We had a 94 Ford L9000, that was doing the same thing. We blew out the vents and had sandwich wrappers, dust, pens and a Bluetooth ear piece come out of it. That really made a difference in the volume of air moved by the fan. Not sure if your vents are sized like those were.