Your engine is supposed to use NO coolant. Coolant is like refrigerant for your ac system, if it is not leaking then it is not “used.”
If you had the truck in there for that repair 10k miles ago and it is the same issue, then they did not fix it and they are responsible for the correct repair. I would escalate this to the service manager, then the dealership General Manager, and then to GM customer service, in that order. Make sure you have the service records in hand when you are doing this. Make sure you are “nice” during this whole exchange to start with. You will get farther that way, but be ready to use the fact that you are a forum member, you have a friend that works for a major automotive publication, Ect, if and when you need it. Lol.
Good luck and keep us in the loop.
2015 GMC 2500 HD Denali, Cooper ST MAXX 35-12.5-20, 5" Rough Country lift, & EGR pocket flares.