Lots of great feedback. Here's what I think I'm going to end up doing (feel free to share your thoughts)...
I'm going to get a solenoid relay isolator (or whatever it's technically called), maybe
this one.
I'll hook each of the batteries to their post, ground the one post, and the final post which normally is hooked to any "on with ignition" I'll do just that but add a three-way switch. When the switch is in full-off it will disconnect power to the relay and isolate the batteries. The other two switch positions will connect on "on with ignition" power supply (so that in that position operation will be "normal", batteries connected when the engine is running) and the final will be connected to a supply that is ALWAYS powered so I can override and force the two batteries to connect (I believe this is what you can call "emergency power")
Does that compute or am I hair-brained?
I'm thinking I'll go this route because
- Solenoid isolators are MUCH less expensive for higher amperage
- Will not cause a voltage drop (a bit important for the more sensitive elctronics I plan on powering--ham radio)
- Many of these solenoids (though not the one I linked to) have warranties for a few years or guaranteed to last "100,000 On-Off cycles"
- This way gives me the control over things and an excuse to add a new switch in the cab. (though admittedly this isn't necessary).