I strongly recommend the book RV Electrical Systems. It costs in the neighborhood of $20, but is well worth the money spent. Even if you understand wiring, it is an excellent reference book to have on hand, for both 120VAC and 12VDC.
I used #6 - 4 wire flat UF (underground feeder) cable for my umbilical. It is hardwired into my electrical box, and wraps up under the bus (well, theoretically, since I haven't built the mount yet) when not in use. It has a solid copper ground wire, but I am not going to wrap it up a) tightly or b) frequently.
You want to use some kind of "mains breaker" breaker box, not a "mains lug". The mains breaker boxes have a breaker (as the name denotes) between the panel and the supply main. Mains lug boxes just have lugs that you clamp the supply wires down with -- no breaker. That is not good from a safety standpoint. Very not good. Lug boxes can be used as distribution boxes in a system, but should not be used as the box that brings the electricity into the system.
My breaker box has room for 20 breakers. I used 8. I installed the breakers with one empty space between them. That made for a much easier wiring job. Plenty of room to work with, plus I have loads of room for expansion if I ever decide to add any circuits.
It's really important to plan your loads before you do your circuit work. You want to make sure that realisticly anticipated loads on a circuit do not exceed the breaker's capacity. For instance, my water heater takes 10AMPS, and shares a dedicated circuit with ONLY a small 20" florescent light. It is on a 20 AMP breaker, and the wiring is all adequate for at least 20 AMPS.
Wire size is critical, particularly for efficient (and safe) 12VDC systems, but also for safe 120VAC systems. The book mentioned above has good charts on the wire sizes and types that you should/should not use for certain applications.
I used a smaller breaker box for the 12VDC system.
Make sure you ground the AC to the bus chassis!