I want to put a small power inverter in my work van?

I just need it to run 1 or 2 m18 chargers and I also want to get a cheap led shop light and put it up on the roof i figured kill two birds with one stone and just plug the light in where i plug the charger in instead of wiring a light and a charger seperate.

Ive never done this before and im seeing a couple different ways to do it. just want a clear simple awnser as far as using a fuse or a breaker switch or both and running 2 wires from the battery or just grounding the neg. to the inside body and how much power will i actually need. also will it kill the van battery if i leave the charger on all night? i would like constant power for the light setup but if the battery chargeris going to kill the van battery maybe i should go with a diff setup.

the milwaukee pulls 6a max
and 18v max
not sure about the light but i would imagine not much more than that.
also trying to go as cheap as possible its a company truck