DRIVING ON INTERSTATE 15 AT APPROX. 65 MPH 30 MINUTES FROM DEPARTURE A JACK WARNING LIGHT AND BUZZER CAME ON. AT THE SAME TIME A PASSERBY WAS WAVING US OVER. WE PULLED ONTO THE SHOULDER AND EXITED THE COACH TO A FIRE AT THE BATTERY COMPARTMENT/ENGINE AREA. I GRABBED A FIRE EXTINGUISHER AND TRIED TO EXTINGUISH THE FIRE TO NO AVAIL. WE HAD 0 WARNING OF ANY PROBLEMS. ALL DASH INDICATION WERE NORMAL.