I seen Truck Route for I-5 in the Sylmar area at the Newhall interchange that gets used as an alternate route.
At the south end of the San Joaquin Valley there is also an alternate I-5 for Trucks specifically and that's at the south end of CA-99.
The Newhall truck route is a separate freeway in both directions, with a different centerline from the I-5 mainline, and Caltrans treats it as a separate route (route 5S). It's an old routing that trucks are required to use, to bypass the I-5/I-210/CA 14 mess. Cars can use it too, and indeed use it to exit to the Sierra Highway (historic US 6), which has no exit from the mainline.
The truck lanes at the south end of CA 99 are southbound-only.