Trailer from different state to tow vehicle?

We spent a weeklong vacation across 3 or 4 states with such a combination one time; our tow vehicle was involved in an accident several weeks before the planned trip, and wasn't ready by the time we needed it for the trip, so we borrowed our old tow vehicle back for that week. (We had sold it to a friend, who lived just across the state line.) No problems, but I did have her write a note, which we kept in the glove compartment, which said it was ok for us to borrow the vehicle, just in case there had been one.