A guy I work with has a f750 dump truck, and we cannot find manuals for it either. He kept looking on ebay and got a Chiltons, and it helps a little bit, but not much. I think that is the only way you are going to find one. Some obscure used book place on the internet, ebay, or possibly a Ford dealer that has one laying around.
You may also find you can only get one for the body, and may have to find a seperate one for the engine and the rearends and the tranny, since the larger trucks came with many different custom set-ups.