I have read on other forums, the roof-top a/c is expected to cool down a motor home 20 degrees cooler than the outdoor temp. You seem to be doing worse than that. Your ideas all seem logical. Try boosting the "cool-down" with the engine a/c, then maintaining the lower temp using all the means you mentioned.
Our older 2007 PC-2350 without a slide, has thermal windows. We have not yet tested the cooling like that. But we have benefited during freezing over-night temps. Our PC stays fairly warm except for the cold that enters through the front Ford chassis area. At bed time, we close the bedroom privacy curtian which makes a big difference for warmer sleeping. It blocks out the cold draft from up front, and the furnace then runs based on the thermostat by our heads.
But that data does not help for the reverse. Based on the cold that enters through the front driver's area for us, I would assume the worst of the heat you are getting, also comes from there. I would focus on padded windshield and door window shades that mount outside the glass. I know you mentioned an outside windshield shade, but maybe a padded one would be more effective. Also inserts for the skylight and roof vents. They do seem to radiate the outside temp inward a lot, don't they? Consider purchasing a sheet of non-crumbling foam from a home improvement center. They sell 4x8 sheets (pink or blue colored), 1"-1.5" thick for insulating house foundations from the outside. They are placed against the foundation, then buried. With a sheet of that, you could make a whole lot of perfect fitting inserts for around the RV. Some places sell broken sheets for next to nothing.
Oh, and it goes without saying, draw down your outside awnings. You can extend you main awning just a foot to eliminate much daytime sun. That can be left unattended. We have a dinette awning which helps a lot with direct sun, again can be left unattended if the wind picks up while you are away.
