Ask about a "competitive take away" discount. If you own a vehicle from one of GM's identified competitors - Ford, Toyota, etc. - there was like a $1-2K discount for my '18 IIRC. It doesn't have to be a truck, and you don't have to be selling it. (I own a RAV4 and they just wanted proof I owned one, so they made a copy of the registration.) Not sure it was timing sensitive, like near end of model year, or regional, but they offered it up in addition to my Supplier discount.
More than one dealer told me supplier is the best discount available, outside of the GM Employee pricing.
2018 Colorado LT, Jet Black Crew Cab Short Box, V6, tow pkg, NAV, Rhino bed liner, Husky Floor liners, HomeLink mirror