The best way to make sure video is preserved after a truck accident is for the police or state patrol to pull the dashcam memory card at the scene. If they do not, then you must make sure the trucking company is saving the video.
Getting a semi-truck accident attorney right away can help. An attorney can send a detailed “preservation letter” outlining everything that the truck company must preserve. Once a preservation letter is sent to the trucking company and its insurer, they will be required to keep the video safe. If they do not, the fact that the video was destroyed or lost may be able to be used against them later.