It appears that the Fonda Diary is available at the Special Collections Section of Western Illinois University http://www.wiu.edu/library/units/archives/archives_web.sphp?id=103
but it doesn't seem to be available online anymore.
I know this is probably not the information you are looking for but he also recalls his service near Port Hudson in a letter written after the war at http://www.mikemedhurst.com/Image_pages/FondaALS.html
I hope this helps.