I have picked up a C-Gen generator with I believe the same Kubota engine, Not positive but have heard same engines from the same plant some painted yellow?. I have a pile of wires for a future project to sort but anyhow I tried to see the alternator no. but tag is to far gone but did notice a ND stamping and take that as nipondenso, ( possibly call a tractor salvage yard). as 800.00 is gold money in my book and way out of line . Do you have a picture of that Gen.
When the gen is running it is not charging the start batteries, and they have a draw from some items in the bus that is not wired thru your whatever the generator is powering - inverter, house bats, etc.. I think im on the same page with ya.
I need a new one but a digital volt ohm meter " having min - max readings" capability is super and especially walking a 40'bus, and or a helper watching the readings while testing each of the possible gen start wires when in the bus buttons or whatever you might have for remote switches are operated.
Im working another project now but am very interested in your wiring as mine is I think very much the same yet everything was cut.
You say you received a customer wiring pdf, that would show several things like motor items. Some don't put to much other because all bus builders are different or use other brands.