Why does the ac not blow hard on my 359 peterbilt?

Where does it take the air in when it is on recirculate?|||90%+ intake on floor, 10% or Less from outside. Possible Weak/Bad Evaporator Fan.|||Most that I've seen is just below the windshield under the hood. I would be looking for a air filter in the heater box if your not getting much air movement. The owners manual should help with it's location.