You should be fine with that system. I would gauge the wire to the load, plus a bit more. The LED lights draw very little current, so small wire is good (14-16 ga). The USB ports will not be drawing much current either, so small wire again will work. The fan I run (MaxxAir 10 speed) uses about 4.7 amps on high, but we don't run it that fast. 16 ga is good here as well.
The main draw will be the 12 volt sockets. They can draw up to 15 amps each, so a heavier wire for each one would be best. Wire is figured as a function of load & distance for 12 volt stuff. Here's a good chart to look at for your stuff.
http://www.rcgroups.com/forums/showthread.php?t=2132883I would just look online for a 12 volt fuse block & go to a home center store to get a grounding buss to use to tie the grounds together (it's just a metal piece with some holes drilled in it). Basically, you are running the positive battery lug to the connection at the fuse block for feeds & the negative post to the ground strip. Remember, when selecting fuses you are protecting the wire that runs the circuit. Run the hot out & the ground back from each circuit.
I would use 15 amp fuses for the 12 volt outlets & 5 amp fuses for the other stuff. 5 amps is OK for the fan, typically.