I know it is illegal in WA. An out of state RVer was pulled over for towing a boat behind his fifth wheel on I-90 while heading west. The patrolman allowed him to tow his combination off the freeway and then tow one vehicle at time out of the state instead of calling a wrecker. I didn't ask the driver stopped at the gas station whether he was fined or not since he was not in the best of moods upon getting the news. Actually I didn't even have to ask what happened - I heard him spewing across the parking lot over the sound of diesel trucks...