As already said, contact Freightliner for the chassis GVWR and GCWR. The chassis tow capacity is the difference. Whether HR used a hitch receiver equal or better than that is a different question, but chances are very good they did. There should be a hitch rating on the receiver itself, but it may have been a sticker that is gone by now.
Call 1-800-FTL-HELP and have your chassis VIN handy.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL