The California Highway Patrol says it broke out on Highway 37 just West of Lakeville.
The fire forced emergency responders to close westbound lanes until crews could get the fire out and remove what's left of the trailer.
No word yet on on possible injuries or how the fire started.
The CHP shared photos on Twitter and Facebook of the fire shortly after 9:30 a.m..
They show the trailer nearly fully engulfed in flames.
"Both lanes are blocked by a travel trailer fire (no animals inside). No estimated time to open," said CHP Marin in a Facebook post.