I HIGHLY recommend buying the current edition of "Traveler's Guide to the Firearm Laws of the Fifty States" because it very SIMPLY explains the carry & transport laws of firearms for every state, depending on whether you are a resident, a concealed-pistol-license holder, or a visitor. It also contains a list of Contact Agency information (addresses & phone numbers) which I recommend that you keep handy. Not only should you reference the book for the local gun laws before entering any state, but it would also be a good idea to call a local agency that can double check the CURRENT laws. It is no surprise that laws are constantly changing, often for the worse.
All of these tiny details make a big difference, depending on which state line you cross and what you intend to do there:
While in the state, are you going to leave the gun in your vehicle?
Do you want to conceal carry OR open carry in that state, outside your vehicle?
Do you have your CPL/CCW? (concealed pistol license/concealed carry weapon permit)
Are you a resident of the state where your CPL/CCW was issued?
What do you have in your vehicle: a rifle/shotgun OR handgun (or both)?
How long do you plan to stay in that state?
Are your firearms loaded or unloaded?
Are they visible in your vehicle or hidden from sight?
Where are they in your vehicle? It often makes a difference whether they are they encased, secured, and locked in your truck (or a rear compartment) OR in your glovebox OR on your person OR within reach or yourself or a passenger.
Now you can see how important it is to pay attention to all the little details regarding your firearms that are inside your vehicle while you are traveling . . . In one state, it might be legal for you to have both loaded rifles and handguns within reach, while in a neighboring state, handguns may not allowed in a vehicle at all while rifles may only be allowed unloaded & encased in the trunk!
Again, laws constantly change, so call local agencies for the most up-to-date info. You can also check out this site: STATE GUN LAWS (NRA-ILA)
Be smart and be safe!