Try buying some insulated thermal curtain panels. They look like regular curtains, just a finer mesh that resist heat transfer across them. You can find them in places like BB&B. I put these over my larger windows and hold them tightly in place against the window frame with small but strong magnets at the window frame screws. This traps in the heat that comes through the windows and the aluminum window frames, and significantly reduces the amount of heat flowing into my RV from sunlight. I bought the sheer white curtains to keep my RV from becoming a cave. It has worked well for me even on days that are over 100F.
https://www.bedbathandbeyond.com/store/product/smart-sheer-trade-insulated-crushed-voile-sheer-window-curtain-panel-and-valance/222114?keyword=insulated-curtain-panel-sheer
