Dual battery alternate wiring options

[davepl]

My truck has the dual battery option, and looking at the wiring diagram, it seems to work thusly:

1) When the key is ON a relay closes that connects the secondary battery into the system. Both batteries will charge.
2) When the key is OFF the relay disconnects the secondary battery. This way if your trailer/camper is connected to the secondary battery, you can run it down without affecting the main battery.

So I have a couple of questions:

a) If the relay is hot during cranking (and it takes its power from the IGN UNFUSED line), then you'd think the alternate battery would be connected during cranking. This clearly should NOT be the case, as the wire between them is maybe 8 gauge and just couldn't do cranking current. So is IGN not hot during cranking, or do I misunderstand something else?

b) Since I'm not using a camper, I'm wondering instead if I should hook it up like a marine system, where I could run off A or B or BOTH. You have to then charge them through a battery isolator (giant diode). I think a diode causes a 0.6V voltage drop, but not sure if this is a big deal.

I'm thinking I'll stick with the completely stock setup and if I need to boost myself I'll just keep a pair of cables under the seat. But I'd be curious if others have come up with interesting alternate configurations.