FWIW, I think it's very fair. I would love to have it - I just really have no use for it.
In 2017, I picked up a 2003 Yukon that was relatively clean - needed tires and some other odds and ends. 142k miles. Got it for $6k, and I thought it was very clean for that price....seems like anything nicer in a GMT800 has the price exponentially skyrocket.
My point is, you're asking double but essentially for a new truck. I ended up putting new tires, a lot of other little odds and ends, complete brake system (hard and soft lines/calipers/rotors) and fuel pump in it, and it's not nearly as clean as what you have.
If you're able, just hold off for the right buyer, and it'll sell. I think it's worth every penny.