All I can add is the standard RV colors for 12VDC wiring is BLACK is HOT +12VDC and WHITE is FRAME GROUND -12VDC
When I am running wires to the outside I like to do this from the pass-thru storage area where I can get a good view of both sides of the wall.
For speakers you probably want to use a lift lid type box where your connection sockets will be out of the weather.
My POPUP trailer has weatherproof type built-in speakers that are mounted on the outside wall and accessible from the inside in the back of a cabinet... ALot of folks like to use a wireless type speaker connection for the outside and not have to run external speaker wires. WALMART should has a good selection of wireless stereo speakers that might work for you... CC CRANE has a wireless transmitter that you can plug a stereo signal into it and broadcast out on a FM radio channel which does pretty good. Available thru AMAZON...