Need some help

[littlebigrig6.0]

My fathers 99 E350 7.3 has had a starting problem for a little while now, He used to have to start it on ether for a while, only when it was cold, well now he cant start it at all, the truck goes to turn over and it sounds like a weak battery, replaced the batteries and the starter and still go to turn it over and it gets weaker and weaker. Any ideas what could be up with it,

[littlebigrig6.0]

Ok update, checked the batteries once more, had one bad battery took it back replaced it and now Im getting the starter to turn alot faster, BUT still no start, I replaced the glow plug relay, I have an audio file of me trying to start the truck if any one can help me figure out how to post it.