ENGINE FIRE - WHILE TRAVELING AT 55MPH AND TOWING A FIFTH WHEEL TRAILER ON THE INTERSTATE, THE VEHICLE INDICATED "LOW FUEL PRESSURE" ON THE DASH PANEL. A FIRE ERUPTED IN THE ENGINE COMPARTMENT SENDING SMOKE INTO THE PASSENGER COMPARTMENT. AFTER STOPPING, THE FIRE SPREAD TO THE REMAINDER OF THE TRUCK AND THEN CAUGHT THE FIFTH WHEEL TRAILER ON FIRE. BOTH THE TRUCK AND THE FIFTH WHEEL TRAILER WERE DESTROYED IN THE FIRE. I BELIEVE THAT A DIESEL FUEL LEAK CAUSED THE FIRE AND RESULTING DAMAGE. BECAUSE THIS FIRE OCCURRED WHILE THE VEHICLE WAS IN MOTION, IT LED TO A VERY DANGEROUS SITUATION AND SHOULD BE INVESTIGATED.