Not to take anything away from Marty Moates, but...
I guess Marty Tripes who won the 250 class in 1978 doesn't count?
The Flying Burrito, as he was called in those days before political correctness, trained with Joel Robert in Europe, did well over there, and yes, won the U.S. Grand Prix of Motocross in 1978. Many, including Bob Hannah, consider the big man the most gifted American rider ever.
Unfortunately, his refusal to play the political game (Honda doesn't appreciate when you put your team logo over the Honda logo on the tank even when you win the moto) and his inability to control his appetite made his career all too short.