Are you sure it's cooler? Or is it just that you're up there at night when the temps are normally colder? The reason I ask is because heat rises and normally a cabover space is warmer than the rest of the RV.
Do you keep curtains closed to separate the cabover from the rest of the RV? That could be preventing the heat from getting up into the space. Try keeping the curtains open day and night and see if it makes a difference.
Replacing the metal blinds would likely help - perhaps just in a psychological way. Might try using a reflective blanket under the mattress - reflective side facing up to hold your body heat in the cabover. I would try one item at a time - otherwise you could spend money doing it all only to find out it's too hot at that point.