My bus is a dark navy blue, and it gets very hot in the summer. Even with an air conditioner running all night, I usually woke up sweating. And if I didn't have electricity...forget about it.
One thing to keep in mind is your sleep schedule. We don't crash until 4ish, so when the sun is blasting strong by 8 or 9, it tends to piss you off. If you're in bed by midnight, though, you'll be a lot better off.
One thing that helped immensely was painting the roof white. I got some thick paint from Menards that was designed for mobile homes. I forget how many gallons it took, but it wasn't too bad. I think I spent $30-40. I just rolled it on. It has helped a lot.