As Jon mentioned, a well performing OTR system can make your teeth chatter.

Another tip is to check the outside air intake flap valve located in the evaporator bay. The OTR system draws air from both inside the bus as well as outside via flexible ducting attached to the evaporator bay door. There is a manual flap valve in that ducting which can be closed, thereby limiting the amount of "outside" air being drawn in. By maximizing the return air supply from inside the bus, I have noticed that my OTR will cool the interior at a somewhat faster pace, but ultimately it depends on how heat soaked the bus is to start with.

Once the bus has cooled down, it seems to continually decrease in temperature as you continue to cool primarily interior air which is now cool to start with.

At least that's how it worked for me...