The contact owns a 2021 Western Star 4700 WD110084S. The contact stated that an employee was parked on a hill and the parking brake was activated, however, while the worker was outside of the vehicle for approximately 4 minutes, the parking brake was deactivated. The vehicle rolled forward down a hill and stopped when it crashed into a tree. There was yard damage to a resident's home. No police report was filed. No injuries were sustained. The vehicle was taken to an independent mechanic where the entire cab was replaced. The contact discovered an unknown NHTSA Campaign, however, the VIN was not included. The manufacturer was not notified. The failure mileage was unknown.