Euripedes' Heracles show signs of a character with PTSD, even if there was no diagnosis for it. Heracles gets home from a long time at war. He has been captured and kept as a slave, completed his labors, and now he is coming home to his family. However, Hera does not like him so she makes him go crazy. Heracles thinks he is back at war and snaps. He kills his family. In the play Hera is blamed for causing it but it shows signs of PTSD. Now obviously this is a myth from Ancient Greece but I think it shows that PTSD isn't a new thing despite lacking a diagnosis until recently.
You might be interested in: http://m.oxfordscholarship.com/mobile/view/10.1093/acprof:oso/9780199534487.001.0001/acprof-9780199534487
Achilles in Vietnam by Jonathan Shay
Medical student and not a historian here. This is a question that is asked a lot, and I would advise you to search previous answers from historians. I'm on my phone, so I'm a little limited.
What's important to understand is that mental illness is an evolving field. The diagnosis of PTSD is decades old. If you go back to WW1 (perhaps earlier) you can find mentions of shell shock, which is considered a "predecessor" of PTSD. It didn't exist in the times you ask about, at least not as a diagnosis. As such, you can potentially infer it, maybe, but there's no primary documents that say that the crusaders came back with PTSD.
Another important thing to understand about mental illness is that it has cultural aspects and manifestations. Modern culture (and warfare, for that matter) is very different from those eras. As such, how something like PTSD would present could have been quite different. Or it's manifestations could have been culturally acceptable behavior and overlooked.
I could speculate, but that is not in the spirit of the subreddit. I would suggest looking at the DSM IV or DSM V. You might also get a kick comparing the criteria from earlier DSMs to the present.
We have to be cautious here because PTSD is a modern construct and it maybe culturally dependent and our understanding of it subject to change. However, there are examples in the literature of symptoms that in my opinion are highly suggestive of PTSD. Plutarch's account (in Parallel Lives) of the life of Roman general and statesman Gaius Marius maybe interpreted to suggest that later in life he had PTSD like symptoms. However, Plutarch was born something like 60 years after Marius' death so this is not a firsthand account.
But Marius himself, now worn out with toils, deluged, as it were, with anxieties, and wearied, could not sustain his spirits, which shook within him as he again faced the overpowering thought of a new war, of fresh struggles, of terrors known by experience to be dreadful, and of utter weariness. He reflected, too, that it was not Octavius or Merula in command of a promiscuous throng and a seditious rabble against whom he was now to run the hazard of war, but that the famous Sulla was coming against him, the man who had once ejected him from the country, and had now shut Mithridates up to the shores of the Euxine Sea. Tortured by such reflections, and bringing into review his long wandering, his flights, and his perils, as he was driven over land and sea, he fell into a state of dreadful despair, and was a prey to nightly terrors and harassing dreams, wherein he would ever seem to hear a voice saying:—
"Dreadful, indeed, is the lions' lair, even though it be empty."
And since above all things he dreaded the sleepless nights, he gave himself up to drinking-bouts and drunkenness at unseasonable hours and in a manner unsuited to his years, trying thus to induce sleep as a way of escape from his anxious thoughts. And finally, when one came with tidings from the sea, fresh terrors fell upon him, partly because he feared the future, and partly because he was wearied to satiety by the present, so that it needed only a slight impulse to throw him into a pleurisy, as Poseidonius the philosopher relates, who says that he went in personally and conversed with Marius on the subjects of his embassy after Marius had fallen ill.
Link: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Marius*.html
There are quite a few threads on the topic. This one has some good info, and I suggest reading the rest of the replies to the original topic.
As a side question, do the modern weapons systems, like accurate high explosive artillery or planes dropping tons of bombs, cause more cases of PTSD?
This question is actually asked fairly regularly- you may want to take a look at some of these previous threads that address some of the same questions you are asking.
Are there any indications of combat PTSD in societies like the Spartans, Khan's or Alexander's armies? (/u/sapere_avde's response is superb)