cpet wrote:
http://handgunlaw.us/
^^^this^^^ has always been helpful to me. USCCA also has info, but I'm not sure if you have to be a memeber to access it.
I would caution about asking the police about law on this matter. It's quite clear they many times do not know the law themselves. I know that seems contraindicated by their chosen profession, but the videos don't lie. Your best bet would be to access the given state's Attorney General website. Most of them have info on firearms transport and possession, etc.
Another important consideration is whether the particular state has a "must notify" provision. Many states require you to notify immediately upon lawful contact of LEO. They mean right now, not 10 minutes from now. Should be the first words out of your mouth if pulled over. That trailer isn't your home while towed on the highway. It doesn't confer the same rights until the tongue jack is down and the welcome mat is out. Whether you abide that or not is up to you (it always is, BTW), but if found you didn't, you can find yourself with a very expensive and time-consuming new hobby. Stay safe.