George,
The state the trailer comes from is the one the registering/licensing laws are followed. Not the state you are traveling through. For instance, WI does not register or plate trailers under 3,000lb. So, our trailers don't have plates on them. Since we have WI plates on our cars the other state law enforcement wouldn't even question it. For our customers headed home with other state's plates on their cars it is OK too. WI law states the trailer follows WI licensing law when purchased here until licensed in the new owners state.
Can law enforcement pull you over for it? Of course. I have seen it happen a total of three times. Once because the trooper (not the customer's home state) wanted to see why there was no plate on the back. Looked at the trailer's paperwork and said everything was in order and sent her on her way.
The second time another customer had just crossed into her home state and the trooper saw the car's plate and no trailer plate and pulled her over. Once again no ticket because she was still legal but since it was in her state he had to give a warning that she was going to need to license her trailer. Nothing wrong with that. These first two situations are why we created the cool custom CAMP-INN plate that comes on your trailer when you pick it up. That way the lack of a plate doesn't draw attention.
The third time was just stupid though. The customer was from CA and went to VT to visit family after picking up their new camper. Great idea and many of our customers do "round about" trips home like this. A VT police chief fined them for not have a plate. They called us, we looked up the appropriate WI law, CA law and VT law covering what it should have for licensing in this situation. All showing our customer was in the right. Faxed it to them at the library I think. The customer took it into the police station the next day and showed the chief. He stated he didn't care and would fine them again if they stayed his town that day. So, what the chief did was illegal and really could be considered extortion. Just going to prove there are always going to be bad apples out there messing with you day. Can't worry too much about it.
Cary