I would suggest that you have a point of excessive resistance, thus the voltage drop trying to drive the current through the resistance. In my case, the nut on the positive lead right on the starter had worked itself loose.
After I removed the generator from the motorhome and took the cover off, the lead was just flopping on the bolt and the nut had backed off quite a ways.
Tightened it up and all was well.
It's not easy to take the generator out, but not really difficult either. I borrowed a motorcycle jack and that made it really easy. Hardest part was pulling out the generator output wires from their junction point.
