I can't find the letter but I did find some notes. Marsden started out in the Infrantry in Charles F. Fenner's Company A, 1st Louisiana Battalion. He was enlisted by Captain James W. Strawbridge on 15 APril 1861 Discharged on 5 April 1862. He reenlisted on 18 August 1862 and was a Sergeant in Company A, 2nd Louisiana Cav, Capt James M. Thompson's Company Vincent's regiment Mounted Partisans Rangers. The letter I referred to was from the Confederate Adjutant Generals office in Opelousas dated 19 May 1862 to Jeff. Davis.

One of Marsden's brothers, William Simpson Campbell (4 Dec 1836 - 9 May 1909), enlisted in the Confederate Navy, serving on the gunboat Pontchartrain (also known as the steamboat Lizzie Simmons). After the gunboat was sunk in Little Rock Arkansas he ultimately joined the 2nd LA Cav. He was taken prisoner at Arkansas Post and sent to Johnson's Island where he remained for 6 months until released in a prisoner exchange.

Another brother was James Green Campbell (12 Dec 1839 - 8 May 1882) and I believed he served in the 2nd Cav as well but have been unable to confirm that.

That is about all I know

David