Somewhere along the line I think I lost track of what bus body your bus has. While the 3800 chassis was used by AmTrans, Blue Bird, Carpenter, Thomas, and Wayne the way the different companies integrated the bus body with the chassis was very different. If it is a commercial body like Turtle Top, Eldorado, or Champion they did stuff even more different than the school bus body OEM's.
If you have no power to the side panel, as in nothing works regardless of the key position, then something is preventing power to get there. Thomas almost always had the side panel powered by a solenoid operating by the ignitions key. Most of the other bus bodies had a live power feed at all times--everything in the side panel would turn on regardless of which position the ignition key was in. Commercial buses differed as to how those panels worked as well.
So the first thing you need to discover is how power is sent to the left panel. Somewhere there is a large cable that comes from the hot side of the starter or directly from the battery. Somewhere along that cable there is going to be a very large fuse, circuit breaker, or solenoid. You are going to discover that somewhere along the way from the power source and the left panel there is something that has stopped the juice from flowing.
If it is powered up by a solenoid/relay that is activated by the ignition key then you need to determine if the solenoid/relay is bad or if the ignition switch is bad. Use a jumper wire and go from the hot side to the activating terminal. If you don't have juice at the left panel then your solenoid/relay is faulty. If you do have juice at the left panel then somewhere the signal from the ignition switch isn't reaching the solenoid/relay.
If the left panel is supposed to have power regardless of the position of the ignition switch then you need to find the break in the power circuit. There has to be some sort of fuse or circuit breaker in that line to protect the rest of the system.
After having said all of that, I would suggest your problem is most likely in the ignition switch itself. But before swapping out the switch you really need to determine why the left panel doesn't have power.
By the way, the headlights and turn signals are stock IHC truck stuff and not integrated into the bus body electrical system. They work independently of anything in the left panel.