Hi, again, Vanessa. It is pretty hard to tell someone how to solve a particular problem without seeing the whole layout, counting ducts, etc., but I may have something that will help.
A ceiling fan in the top floor, with adequate roof vents might go a long way toward solving the problem. That will let the rising heat work for you. By the way, the new ridge vents are a lot more efficient at keeping air circulation going the right way than gable end vents. Any way to vent air at the top of the roof beats gable end vents. A ceiling fan is not real expensive and some good nights sleep makes it worthwhile. The cost of adding or changing roof vents depends on what changes you have to make. Van, this is a solvable problem.