I WAS DRIVING ON 8E FREEWAY AT 55 MPH WHEN I NOTICED THE INTERIOR OF THE MOTORHOME FILLING WITH BLACK SMOKE AND THE SMOKE DETECTOR STARTED GOING OFF. BLACK SMOKE WAS POURING OFF THE BACK OF THE MOTORHOME AS WELL. I PULLED OVER TO THE SHOULDER AND GOT MY WIFE TWO GRANDKIDS AND TWO DOGS OUT OF THE VEHICLE. UPON EXITING I FOUND THE RIGHT FRONT OF THE VEHICLE ON FIRE AS WELL AS FIRE DRIPPING FROM THE EXHAUST TO THE GROUND. AFTER GETTING THE FIRE OUT, I DISCOVERED THE VEHICLE LEVELING SYSTEM HAD A HYDRAULIC LINE FAILURE AND THE HYDRAULIC PUMP WAS SPRAYING FLUID FROM THE FAILED LINE ON THE EXHAUST PIPE CAUSING THE FIRE. THE LEVELING SYSTEM IS USED FOR LEVELING THE VEHICLE WHILE PARKED IT SHOULD NOT COME ON WHILE DRIVING ON THE FREEWAY. I ALSO FOUND OUT THE FIRE DESTROYED THE RIGHT FRONT BRAKE LINE AND THE MOTORHOME LOST ALL THE BRAKES ON THE FREEWAY ON THE WAY TO THE REPAIR SHOP.