Crenn,
For what your doing you don't need the "inverter" as listed in your linkage.
You really could do it all from a 12Vdc perspective. In your case the inverter is just a waste of energy, by skipping it you'll make a smaller, lighter, cheaper, more efficient design. Your looking for a DC to DC 2 source converter with auto switching.
I've got a circuit that does most of what you want except check that the batt isn't dead (under volt). It doesn't use any micros. I'm sure that you can find a chip that will do everything you need, I think maxim has a couple of IC's that will do it.
Good luck!
CrazyBillyBob
I think with a little work you could use a microproc to measure the battery voltage, if it's low; trip a load relay off to disconnect the load from the battery.
as long as you build a little hysteresis into the limits it should work just fine. I just don't have a comparator circuit within easy reach.
A comparator is more simple to implement than a HC12. I don't have any spare HC12 chips either.