Seems like you need the belt routing diagram, not the part number.
If you are on a Freightliner chassis, you need to call 1-800-FTL-HELP with your VIN number. They can then email or fax you a copy of the belt routing diagram.
This is what they sent me for my Winnebago Tour 40TD:
Like a previous poster said, check ALL the pulleys. Last time mine broke, it was the AC compressor that seized. Time before that it was the alternator. Time before that it just decided to snap.
I carry some spares now.