Tail lights not working

Could you clarify if it's the turn signals at both ends that are out or only the rear? If it's both ends of the truck then the likely issue is a failed body control module as it controls the turn signals directly. The trailer connections are controlled through it as well, but on a different section of the BCM.
The physical replacement of the BCM isn't hard, it is located behind the knee panel under the steering column. You remove the trim panel and release the electrical connectors to remove it. The new one snaps in and gets connected. The catch is that unless you have a scan tool with a programming dongle and the connection to GM's TIS program is isn't a DIY type of job as the GM system in these isn't plug and play. The VIN coding has to be updated to allow the anti-theft system and options to operate.

https://www.2carpros.com/articles/how-to-replace-a-bcm-body-control-module

So all front lights, how about rear marker and license lights?
If the rest of the rear lights work and it's only the rear turn/brake/hazard lights out then it's the BCM. If the other lights (marker plate) are out on both sides as well then it could be the rear ground.