If your exterior walls are Filon, then you can’t “open up” the interior walls. The walls are sandwiched together with rigid styrofoam type material and it’s all “vacuum bonded”. Exterior color will play a big roll in how much the interior heats up. It’s not just Keystone that uses this type of construction, it is industry standard.
Best advise I can offer is try park to minimize exposure to the sun, early start with the AC and keep shades down in the heat of the day. I have used windshield foil shades on windows in direct sun and they have helped.
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.