Dear Mr. Moore,
Thank you for posting your question on History Hub.
An individual's Official Military Personnel File (OMPF) is the best resource to obtain medical and treatment records; such records were not copied and stored separately at another National Archives facility.
However, because the veteran's records were destroyed in the 1973 fire, other information concerning medical treatment may be located among the individual medical treatment records retired by military hospitals which are in the custody of the NARA's National Personnel Records Center (NPRC). Clinical (inpatient) records are compiled when members are actually hospitalized while in the service. An overnight stay or admission generally makes a patient an inpatient. The resulting records from hospital care are called either clinical or inpatient records. Clinical (inpatient) records are filed by the name of the hospital in which the member was treated. Therefore, please send the name of the hospital, month (if known) and year of treatment, as well as the veteran's name and social security or service number to NARA's National Personnel Records Center (NPRC), (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. Some record collections date from the 1940s and 1950s, but more comprehensive information exists beginning in the 1960s. These clinical records are a separate series of records and distinct from official personnel files).
You might also request that the NPRC check morning reports of the 66th Truck Battalion for the date on which he was injured in their custody. The morning reports are arranged by unit number and date, but will indicate only that an individual was on sick call. They will not provide medical data relating to that individual.
The National Archives and Records Administration