If you are driving.... you will pass through many states. The laws can obviously be different from state to state or county to county.
I would submit that the safe route would be keep the gun unloaded and in a locked container or trunk to be safe.
I do not know of any law that says you must tell what is in your car.
There are many laws covering what is not permitted so be sure to brush up on them. If you tell us what you plan on doing we can guide you better.
There is no law prohibiting OCing but people may call the police on you. Most departments know OC is not a crime but you may be approached.
Cooperation is your best bet if you are contacted.