There is no simple way to make the original power on light in your panel operate with your new converter. It is easy though to get a simple neon night light and plug it into an outlet to see if the power is on. If you really want the original light, I know there are some posts here on the Forum as to how to do it, but most think it is now worth the effort.
The "chasis bonding lug" should be attached with a good sized wire (I would use about a #10) to the body/frame. Both the 120 volt grounding wire and the negative 12 volt
wire seem to do the same thing, but the manufactures also include that bonding lug, and I think it is a good item to use, as another "just in case".
There are probably some RVIA regulations which cover the exact size, I don't have that information available.