[Carl n Susan]

Ah ha I found the Post

http://www.montanaowners.com/forums/....php?p=1142356

As Gary subsequently learned, the 2017's and up no longer use the 12V charge line relay and circuit breaker. Their positions in the fuse panels have no circuitry to connect to. After several trips to the dealer, they finally learned about the new improved system.

All 2017 F-Super Duty vehicles equipped with a factory trailer brake controller (TBC) utilize a trailer module (TRM) which initiates the trailer charging system. For trailer charging to operate, the TRM must detect a trailer is connected and a brake pedal input before charging will be present at the trailer connector. Adding the trailer battery charge relay and fuse into the battery junction box are not required to operate trailer charging with a TRM. This is covered in Ford document SSM 46257.

Ref: https://www.etrailer.com/question-278872.html plus many other internet sources.