Can you either (1) fill out your profile with engine and transmission info or (2) post this? Also can you describe the symptoms in more detail?
Is the starter spinning the engine every time it's supposed to? If not, have you tried messing with the shifter as you try to start the bus? (Sometimes the neutral safety switches get out of adjustment or wear out). Do you have door and window safety switches that prevent starting if something is open/locked/unlocked? (One might be making poor contact?) Is the starter turning slowly? (Assuming new batteries, there may be corroded connections?)