I always put a curtain up to separate the front drivers area from the rest of the bus much easier to heat and cool. The roof panels are all scews and perfect for attaching a cloth strip or the edge of a sleeping bag. T 20 type screw head and yes I have a big old milwalkee drill that makes it an easy job. Some times when its so cold and the wind blows so hard, the bus can seem a little drafty. Oh ya what Smitty said about the fan - and rugs on the floor for the winter.
When you have a 1 wire 12vDC light, you have power going to the light and the ground is a chassis ground. You are grounding the light when you attach the light to the metal of the vehicle (bus) with the metal screws.
You have to have a common ground. You can ground your house batteries to the chassis and then you do not need the ground line to the lights since the lights are grounded to the chassis and the battery bank is grounded to the chassis ( chassis = common ground).