Thanks, I’ll try replacing everything and see if that fixes it. The belt might be nearly worn out. I can keep the old belt as a spare. Last time I changed a belt, tensioner, and idler it cost $115 in parts. And I didn’t know to clean the pulleys. Any residue affects the belt.
With a ex-school bus it’s tricky to look up the alternator as it might not be stock. As with ambulances, shuttle buses, etc, they often use a different alternator.