No 12 volt power question

[Baby Blue]

1998 maxum 2800 scr
454 single engine carbed
749 hours

Been a week since we were at the boat.
Turned the blower on and ran for a second then nothing.
Super faint beep at the key then nothing . No interior lights, just a weak light at one of the helm switches.
Pulled battery from other boat and hooked up but still nothing.
Thinking battery switch or connection of main power to starter (possibly corroded? Have to check)
Have a 2 bank charger that shows both batteries fully charged.
I only have a light tester at boat now. Light on with battery 1 and battery 2. When I turn off both batteries I get intermittent light power out. Thinking switch is failing but surely enough power on other side of switch for something to come to life.

Question is this:
I haven’t traced it yet but does the power cable come out of the switch and directly to the starter and from there branch out to the rest of the boat.
Is there a main fuse somewhere besides the small blade fuses under the helm.
I’ll trace the ground when I have time to check out. Is it under the engine?

Thanks for any help provided
Mark