TRUCK WAS PARKED ON STEEP DRIVEWAY/PARKING LOT. EMERGENCY BRAKE AND WHEEL CHOCK WERE SET. AFTER APPROXIMATELY 5-10 MINUTES TRUCK BEGAN TO ROLL DOWN HILL INTO GULLY. BRAKE ADJUSTMENT NOT TIG HT ENOUGH. CHOCK SET ON ICY SURFACE. PARKING PROCEDURE-ORDER OF ACTIONS TAKEN. EE WAS IN BACK OF TRUCK; HAVING JUST PLACED MATS IN THE TRUCK. DAMAGES TO TRUCK APPROX $1,000, EE - UNKNOWN.