Luckily the trailer and motor home parted ways just when the rig reached the top of a hill and speeds were relatively low. The trailer miraculously came to rest when the trailer tongue buried itself into a a big pile of sand off the shoulder of the highway -- the boat didn't even come off the trailer. The tongue was a bit bent but the boat was completely fine.

We were following the rig in another vehicle and saw the whole thing, including the sparks coming from below the boat trailer before the motor home and trailer separated (I assume the safety chain was ground through somehow). The motor home carried on for about another quarter mile until they realized the boat trailer was no longer behind them.

This happened when I was 14 (many years ago) so some of the details may not be entirely right, but it was one of the crazier things I have witnessed.