I bought a 91 f350 in May and now I am having trouble starting it. It was having a hard time starting and if I kept hitting the pedal it would start up and it would blow white smoke the whole time, once it started up I had to keep the key turned on and keep the idle up or else it would die. I replaced the batteries and it seemed to do the trick but then it did the same thing. I recently tried to start it and it wouldnt go at all and cranked really really slow. My batteries both read over 12.5 but I did not do a load test. Not sure what my first step should be. I feel like the starter could have been on the way out but I dont want to jump in and buy a starter just to find out its not the issue. It could have been air in fuel and then the starter could have died from me trying so hard to start it.

 

if you can count the engine turning over the starter or cables are shot.
click on the picture to play the video.
here is my 88 with a bad starter at 70 degrees:


and here is the same truck with a new starter at 40 degrees:

 

You replaced the batteries. I assume they were new, but, I'd pull them and have them load tested just in case, one could have a dead cell. #2 would be as suggested to look at the starter and the cables.
The 1st thing I do is put a volt meter on the batteries and crank it over, if it pulls down to 10 volts or lower then I would have the batteries checked and I'd remove the starter and have that bench tested. If you're holding the key in the start position when the engine is trying to run, you've probably destroyed the starter and over heated the cables to it also. When have you check the glow plugs? maybe it won't fire because they aren't working properly, maybe they aren't getting hot enough to fire?