[prairieschooner]

Jeff,
Use a Volt Meter so that you can see what the voltage is at the starter, put the leads at the same terminals that the starter gets it power from (+) at the positive cable connection (like you stated with a loose nut on a terminal) and (-) at the negative cable connection. Now go back through the tests using the engine batteries and the merge function.
So many times it is simply a loose connection and mostly a loose ground (-). Not sure about your engine but my 6V92 requires 2000 cranking amps and that is why there are two 8D batteries.
Good luck, I know that it can be frustrating, just write down the findings while performing the tests with the volt meter and test the voltage at numerous points of the positive (+) and negative (-) cables one cable at a time.