At this point, since you are the customer, have them check the battery/fuse, cables, and all ground connections. If you are not happy with what they come up with, let them and GM know!
They have seen this problem before, the trucks have been around 6 model years now, should be no mysteries when these problems come up.
And welcome to the site, sorry your first posts are on the negative side, they are good trucks when you get the bugs sorted out.