Do you have some sort of window covering in addition to the factory shades? If not, try the reflective "insulated bubble wrap" stuff that you can get at any camping supply store or on line. It made a huge difference for us.
2014 GMC Sierra 4x4 crew cab with 6.2L and max tow package
2015 Autumn Ridge 329BHU
2014 JayFlight Swift 264BH - SOLD!