Thanks for the answers Numbers, I did a lot of searching here and couldn't find anything about the wire and switch size for the relay. I even called a store that sells relays and he at first said 8 gauge wire, then changed it to 18. He made a reference to the ignition, and the fact that it uses 8 gauge wire and is a 10 amp. So I guess I'll be fine with a switch rated for 20 amps.
I found a pic of how a starter is wired to a relay, let me know if this is how I would wire my relay for the PSU and inverter. Pic is here:
Wiring, fuse, amp, relay questions
Now that I've figured out how the isolater, fuses, and relay is wired, I can think about a distribution block. I've never dealt with car power so this is all new to me. I looked at a pic of a 400 watt inverter by Coleman and it had battery clamps on the ends, so I'll have to do something about that.
From my second battery will be a relay to cut all power, then a block I guess.
Thanks again for all of your help.