Okay, let me add a litle complexity here. Let's say I go with a dual battery system on a 94 FZJ using an isolator. Then I want a 3 volt circuit coming off of the second battery, at 20 Amps. Actually, I could add the circuit without the second battery to start out with; that might be better for me right now.
Do they sell some device, voltage divider, or voltage splitter to do that? Where would I get that? It'd be okay if it were 6 volts but 3 volts would be better. My web searches seem to indicate no such thing and I'd have to design my own voltage divider out of some high watt resistors. How about the alternator; upgrade that in this scenario?
I'd probably want an in cab switch also. Good source for electronics stuff? Is it Mouser.com?