on 10/05/2018, Bill Wright supposed :
I helped a friend (a friend in need...) who had a similar problem in a
motorhome. For some bizarre reason the starter battery was in a locker
instead of in the engine compartment. It wasn't all that far but during a
start the voltage at the starter motor was about 6V. In the end we put the
battery back where it should have been and that fixed it.

Many people do not understand volts drop and just how serious a problem
it can be at such low voltages. They base their calculations simply on
the current rating of the cable and even the pros ignore volts drop..