The chassis builder installed that part, so a call to Freightliner 24/7 Customer Service line should get some help. Plus you can buy parts direct from Freightliner if that stock that one. Have the last 6 digits of the VIN handy and call 1-800-FTL-HELP
There are many sources for such parts, including Ebay & Amazon, but you need to identify the specific part first. If there is a part number of art manufacturer name on the idler, that might be sufficient. If not, call Freightliner. Or maybe a Freightliner dealer could help, but be aware they are independent franchises and often not well versed in the motorhome chassis (they are truck dealers, first and foremost).