lets break the bus 120 volt electrical system down so it is very basic.
So you want to plug in an extention cord that feeds power to some outlets in the bus. That is no different than plugging in a power strip (or surge protector) it's just that the outlets in your case will be more spread apart.
Someone will correct me if i'm wrong....To get your polarity correct, look at the outlet. you'll notice that the standard plug from any appliance usually has one spade that is bigger than the other. The smaller one should be connected to the black wire on the outlet. In this photo, the bigger spade is on the right
once you've ran all the wires from your various outlets to one common place, you can now find a an extention cord that is suitable to run all the appliances in your bus.
I would suggest at least 12 guage, and no longer than 25 feet. Cut the female end off of the extention cord, and connect the white/black/green wires inside the cord to the proper colored wire inside your bus.
Does that make good sense? A good place to keep the cord when not in use is wound up inside the battery box, if there is room.
As for connections, i do not like wire nuts.
They are the most common means of connecting 2 solid core copper wires.
I think that in the skoolie enviroment it is far better to solder each of the wires together.
Buy a good soldering iron for at least 20 bucks, you'll regret it if you buy a cheaper one.
feel free to ask questions