I had the same issue, 2018 FW
In my case the fuse was also okay under the driver seat. Behind rear bumper next to water tank up in the frame there is a plug in relay in the wire bundle run.
Tested the relay and it was bad.
Replaced the relay and the running lights are working again.
In the picture you see the side of the fresh water tank next to truck frame and the relay. Factory zip tie is black.