So I went through this and let me summarize my findings.
Most buses have a steel outer shell, a steel frame in the middle, and a steel shell on the inside. Then they fill the empty space with insulation. But since you have steel connecting the inside shell to the outside shell, you don't have any sort of thermal break. If it's hot outside, the frame will happily transfer that heat to the inner shell, bypassing the insulation entirely.
So let's talk about options.
You could install a thermal break. Basically you would use some sort of insulating material to prevent the outer shell from touching the frame.Are you going to be able to stop metal-to-metal contact on every bolt and rivet? I doubt it. I know I couldn't, so I didn't do this.
You could pull out the inside panels and put a layer of insulation inside the outer shell. This is pretty popular. You lose a little bit of rigidity doing it, but I haven't ever heard of anyone having issues from doing it. Just pull out all the interior panels, rip out the existing inslation, and go crazy with your insulator of choice. A friend of mine used a double layer of blue insulating foam board plus a layer of reflectix and that box really held its temperature nicely! Your bux probably doesn't have a square interior, so you'll end up with wasted space (aka air gaps!) if you use foam board. One good option is to layer the inside with regular fiberglass roll-out insulation and then reflectix inside of that to hold it in.
Or, if you're lazy like me, you can just put reflectix over any surfaces you want to insulate. I'll be honest, I've only done most of the back wall and the windows. If I were starting from scratch, I would put a layer of reflectix all the way around with holes cut for windows. If you're planning to follow springtime like us, it's easy to deal with it being a little warmer or a little cooler than optimal. If you're trying to live in Miami, this won't be an option without a power hookup and an AC unit.