I ran a 1/2 poly water tube from where my tongue box will be to the cabinet in the cabin where all my wires run to, that is where the fuse panel is. It will be to run any forgotten wires after everything is closed up. Worked out well. I also ran a 18/6 cable and 2 16/4 cables (speaker wire) just so I have extra wires if needed at some point. Some will be for a thermostat if we add heat, and for a third break light on the hatch door. I ran some 12ga wire for 120v even though I don't plan to camp where there will be 120v. And now everything we need can be powered on 12v. The computer can charge while we drive, we have an AC plug in the TV.
I used this wire in 16ga for most all 12v, it is great stuff.
https://www.amazon.com/Ancor-Marine-Grade-Duplex-Cables/dp/B000NUYC3S/ref=sr_1_2?ie=UTF8&qid=1523933650&sr=8-2&keywords=duplex%2Bwire&dpID=214cvzxTljL&preST=_SY300_QL70_&dpSrc=srch&th=1&psc=1I didn't run any audio because we are going to use some bluetooth speakers we can move to where we need them, inside the cabin, out by the fire, back by the galley. That way we can have the music where we need it and not have to turn it up where other campsites might hear it.
I wouldn't use anything smaller than 18ga in an area that I wouldn't be able to get to in the future.