Our new-to-us bus is wandering a bit going down the road. Our old bus doesn’t have this issue because we spent a lot of time getting that sorted out, and its steering is tight - we want the same on this bus. The steering is tight enough, and we had it aligned - it just needs more caster to track straighter going down the road and wander less when encountering ruts and so on. The caster values are different for power steering vs manual - it takes more effort to steer when there is more caster - but having zero caster is basically like one of those old style bikes where the wheel was under you and you turned it straight up and down - very unstable - adding caster is like tipping your bike pitchforks outward at the bottom - it makes things track better and more stable.
The GM bus has an adjustment for this, but if your bus is crusty or rusty, you might have some challenges with this exercise. We also have access to a pit, so this makes this work much safer to do - it would be much more difficult and would require additional safety measures such as blocking up the body so that it cannot fall on you and squash you.
Watch the video to see the challenges we had and let me know if you have any questions. Refer to your manual, the adjustment is documented in there briefly as well.