I know the feeling of lack of written data and being the last of the line. There is an excellent book titled "2nd Virginia Cavalry" by Robert J. Driver. It is part of the Virginia Regimental Histories Series. A quick perusal of the Muster Roll in the back of the book shows both of your relatives listed and shows William Fountain as a Lieutenant Colonel and John P. detailed with Ambulance Corps among other things. William was apparantly also a member of the State Legislature and Deputy Sheriff. I also noted that both were absent on sick leave, one in a couple of the Richmond Hospitals, much like my great grandfather of the same regiment.There is a picture of William in the book, courtesy of Mrs. Russell W. Corbin of Vinton, Virginia. It is probably taken after the war since it shows him with white hair and beard.
Will