We ran our wiring through the tubes that are already in the bus for the tail lights. (Above the windows.) Maybe run that to the back, run it down the back corner wall and then under whatever's back there in the floor. For instance, bed or storage space. Then back up the corner and into the tube again.
My build thread:http://www.skoolie.net/forums/f11/build-thread-for-haulin-oats-and-goats-11237.html#post113500
A gal, a guy, three cats, two dogs, one rabbit, and one goat, traveling the country together.