Run the power lead outside, under the van, strapped tight to the frame and clear of heat, always use a grommet when passing through metal, keeps it from getting cut.
I have a battery in the rear of my van, linked to the front for charging the way I explained, the amps for my stereo are under the rear bench in the back, tied into the rear battery, RCA feed goes down the drivers side, against the wall to the amps. Brand name, heavy insulated RCA cables are the only way to go, not cheap, but low signal degradation, and no interference, 18 gauge speaker wire is fine, unless it's longer than 20 feet, you aren't gonna notice a difference with heavier wire.
Be aware to get the best sound, your sub requires a certain amount of airspace and firing in a certain direction for optimum sound quality.