It takes a supply path and a return path to supply any load. Therefore the size needs to be at least the minimum to supply that load. If you have a ground wire servicing two loads, it needs to be heavier then the two supply wires to do the job.
You should be able to use the frame of your truck as the "big ground bus". However, relying on a ground path from a light through sheet metal is folly. The easiest solution is to run a short ground wire from each light fixture to the frame.
