Thanks for all the answers. In looking at the batteries today I believe I have the following setup:
Position 1 = Two Napa marine batteries (https://www.napaonline.com/en/p/NBP8240
Position 2 = Napa Marine deep cycle starting battery (Here's a link to the actual battery: https://www.napaonline.com/en/p/NBP8274
The engine does attempt to start on battery 1 or 2 alone, in that it cranks repeatedly but won't start. The cranking speed seems about the same regardless of whether I'm on one or both batteries.
I measured the voltage at rest and while cranking the engine and got the following:
- Before attempting to start, all batteries were at ~ 12.6 - 12.8V.
- When attempting to start with just position 1 or 2 alone, neither positions' voltage fluctuated much while cranking (Voltage jumped around between 12.6 - 12.8V, but didn't really drop). Neither position started the engine consistently (see below).
In the "both" position the engine always started. Voltage started at 12.6, dropped to 11.9/12.0 while cranking, and went back up to 12.8V after the engine started.
Interestingly, after running the engine for a few minutes, then trying to start in position 1 and 2 alone, the engine did start once from both positions alone. I wasn't able to repeat it after that (tried many times). Unfortunately I wasn't measuring the voltage the one time it did work correctly.
The wiring basically looks like the attached diagram, with the only difference being "battery 1" is the two smaller batteries connected together.
I'm more confused now than ever......