I have a 12 foot garage door. Will the camper on the truck fit? The GMC is a 2500 HD duramax with helper springs and standard wheel rims. The 820 will have a roof top AC
Most likely no. I would estimate that you will be closer to 12'6" but that depends on a lot of factors.
Is this a 2002 or a 2022 truck? The trucks have gotten about 6" higher.
Does the garage have an overhead door? You lose about 6" of opening height with an overhead door. Even if you push it up past "open" it never quite clears the opening.