You have an HD truck which has absolutely nothing in common with the half tons. I suggest you post in the 2500/3500 section so as to not confuse your problem with this 1000 page thread.
Generally you file diminished value not on your insurance but on the other guys insurance. Your insurance has no obligation for diminished value. Whether you can do diminished value is also state specific.
File your complaints here:
https://www-odi.nhtsa.dot.gov/VehicleComplaint/
Don't feel sorry for anyone if you post on this site but don't report the problem to NHTSA. This is a serious safety issue and GM's feet need to be held to the fire. Where is the recall?
Why is this not a safety recall? This is a serious safety issue. I suggest you all report this to the NHTSA
https://www-odi.nhtsa.dot.gov/VehicleComplaint/
Doesn't matter that you can figure out how to stop in this situation. It is a safety issue and if systematic should be a safety recall. Reporting the issue to NHTSA can help force an investigation.
Isn't this covered under the bumper to bumper warranty? I recall some recent threads of battery issues which were covered under warranty. I would contact GM and try and get your diagnosis fee reimbursed at a minimum. Your dealer sounds like a bunch of crooks.
Look on the drivers side door, there will be a sticker that tells the specific payload capacity of your suburban as it left the factory. It will say something like total passenger and cargo weight should not exceed XXXXlb. What does it say?