2000 kw w900 mystery electrical issue

I'm working on a 2000 w900 with c15 complaint only starts with starting fluid. When he shuts truck off crank no start condition then hits it with a wiff of e and it starts and runs fine. I found loose battery connections as well as faulty batteries replaced tightened all connections . I found if you put me in run position and use starter button at starter engine starts right up shut key off and crank no start.
Found loose connections in fuse block to ignition relay . Checked voltage drop when cranking at key drops to 8 to 9 volts, does the same when using starter button but it fires up. Any thoughts or ideas on situation ???