The incident was reported around 3:45 a.m. at Diamond Bar Boulevard.
Video from the scene shows the RV fully engulfed in flames on the side of the freeway, as well as the adjoining hillside. The fire burned about 1 acre of brush before crews extinguished it.
At one point, all lanes of the westbound side of the freeway were shut down. The HOV was eventually reopened but there was still an extensive traffic backup in the morning.
DEVELOPING: We will add more details to this report as they become available.