If your AC blows cold at road speeds, but blows warm at idle like at a stop light, stop sign, or a truck stop parking lot, it's most likely worn seals in the compressor, so the compressor needs to be replaced. Sitting at idle, it should blow cold. If it takes increased RPMs to get it to blow cold, it's a compressor problem.
There are other causes, like too much or too little refrigerant, excessive oil in the refrigerant, obstructions in the lines or over the condenser, even high humidity can cause reduced performance. But cold at speeds and warm at idle is classic worn compressor.
On a Sprinter, you park it in the shade, open all the doors and windows, in Neutral, all duct vents open, and controls set to fresh air (not recirculate), and have the engine idling at operating temperatures. A temperature probe into the dash vent just to the left of the radio should show between 35° F and 54° F while the compressor clutch is engaged. If it's not in that temperature range, you need to have it checked out.
