Because you have a future of dealing with fun stuff like this (obnoxious grounds), the good money is on that it would not be a bad idea to make a ground test cable to store along side your other test equipment.
I'd make it from 10 gauge, which would still be undersized for the alternator and starter, but a quick on and off could still give you clues.
SIDE NOTE:
Many of us use the old battery cable and test light trick to track down problem circuits that drain the battery. That is, after removing the negative cable from the battery, we run a 12 volt light between it and the negative post.
If the light comes on, something is in completed circuit mode, which would drain the battery just sitting.
This can be tested: If the light is off, turn on the park lights, which will create a closed circuit, and the light should come on. If not, you have other issues, like a dead battery, or the park lights not coming on for want of a working fuse or other issue.
SWAPPING the light over to the positive terminal, you can test fully open grounds. You could use the probe to test at various points of a ground termination point (the bolt, the leads connectors, the actual wires).
Just keep in mind, a ground can be just bad enough to stop a high amp item from operating (lights, starter), but a lesser one, like the test light, my still appear to work. So this is a all-or-nothing kind of test.
Backed by a test cable to provide a solid ground for the suspect spots, much can be learned. For example, if it tries to turn over a bit better, the safe money is on a bad ground to the engine."
Oh, and Rick was right, but so were you. I confess I missed that obvious one at first too, because I focused on the fact you could start a rig without an alternator. Good catch, reminding us the starter needed that entire engine grounded. As too often is the case, the devil was in the details.
