So we haven't painted the roof of the bus yet, but we have painted the metal floor, the subfloor (both sides) the walls under the foam insulation, and part of the metal ceiling inside. We've only been able to do the ceiling in the back half of the bus though. We mixed ceramic beads with regular Bullseye primer. The beads we used are Insuladd beads. http://www.insuladd.com/
We first used them when painting the bedrooms in our house as they're either the hottest or coldest rooms in the house, depending on the season. Because they're reflective, they don't make a difference in the summer because we don't have anything outside the house painted, but during the winter, there was a noticeable difference in how long the rooms stayed warm. They were typically getting much colder much more quickly than the rest of the house.
As for the bus, we use a space heater. When it's running, you can put your hand on the painted surface and the other hand on the unpainted surface and noticed a very measurable difference. Literally inches apart, one feels pretty warm and the other feels pretty cold. We also haven't painted the roof yet. So I can't say it'll keep the bus warm when it's cold outside, but I can say it makes a difference. We keep the heater in the middle and heat either the front or back of the bus depending where everyone is. The back of the bus will stay warmer longer than the front of the bus will.
Now, we have all the stock windows, 1" white Styrofoam insulation on the walls below the windows and under the plywood floor, and all stock insulation in the bus was left as is. We do have pretty heavy felt curtains over all the windows.
The reason we add the beads to the primer is because it actually changed the tint of the paint a bit. When we used two different cans of paint, it changed the tint in both cans, but not in exactly the same way so you can tell a bit when one can ran out and the other started being used. I guess if you had a big enough container to every drop of paint you were going to use for the whole job, it would be a big deal, otherwise I'd recommend putting it in the primer. The primer is just white and eventually covered up, so it doesn't matter what it looks like.
I browse the site almost every day and feels good to finally add $0.02.