I did a little checking and James G Campbell had five cards in his 2nd LA Cav Confederate Service Record (CSR), including his signed parole form from June 3, 1865 in Alexandria. He is listed as being a resident of Bayou Boeuf in Avoyelles Parish. He was in the 2nd LA Cavalry F&S as an Asst Surgeon.

In Checking CSRs for Confederate Officers, I located another 6 cards. On March 13, 1861, he was apparently an Asst surgeon in the Company of Capt james Strawbridge of the 1st LA Regulars (Infantry). There is a letter from Strawbridge ordering him to provide professional services as needed by Capt C M Bradford, also of the 1st LA Regulars. There is also a receipt or invoice for quarters in New Orleans in April of 1861 (1st to 31st). According to one card, he was appointed as Asst Surgeon reporting to Genl Richard Taylor on August 16, 1862, to rank from August 1, 1862. The appointment came from the CSA surgeon General and the CSA Secretary of War. He was almost certainly assigned to the 2nd LA Cavalry by General Taylor in Sept or Oct of 1862, but that information is not in the files.

In the CSR miscellaneous files, I found 1 card indicating that James G. Campbell, a Physician from Opelousas, was to rank as a Major.

In looking at a genealogical site, I did find a listing of William Simpson, James Green and Marsden Campbell as sons of Dr. Marsden Campbell, Jr. I just didn't find them in the census documents of 1850 and 1860.

Hope this helps

Don Parker