2016 Chevy Silverado Diagnostic shows no errors

I purchased a 2016 Silverado work truck close to 2 years ago . Ever since , mysterious “ errors “ would start occurring such as “ check trailer braking system “, “ hill assist on” ( at a red light on flat surface” , “air pressure low “ ( nope I checked it and it was fine)
Best one : truck randomly shuts off when making a turn . First time I went to a mechanic , they purchased the “codes” from GM and did the diagnostic . Cleared a bunch of codes .Appeared everything was fixed , months later , it started again. Flash forward to many distraught conversations with a variety of customer ambassadors “ and final say is it is not under warranty so I will need to pay for another diagnostic. I already did this in the summer and service manager said bc they can’t replicate what the truck is doing , they can’t fix it .
I understand warranties but this was occurring with a truck that was only 3 years old. I haul a horse trailer with this truck . No one seems to be concerned about how it shuts off randomly . Now I drive it ready to react in case it loses power when I’m hauling my horse.
GM refuses to take the truck and fix it .
Now what ? Many computers within the truck in different areas soooo…..
This was the service managers response . The level of frustration for me has reached an all time high . How can GM think this is acceptable ? I even said I would leave it at the dealer for them to drive until it happens so they can see what I am speaking of . I was made to feel that I was making the situation up bc it is so random . I used to really love GM .