Mako there is an article by Alexander Brown entitled "Epicurus on Truth and Falsehood" which I can upload here (or you can get on JSTOR) if you like. It focuses on the details of some commentary by Sextus Empiricus which bears on "truth." But I'm not really sure that I recommend that to you, however, as it might be more technical logic and hair-splitting than you are interested in reading. After glancing back at it I had a hard time finding a passage that jumped out at me as being a clear statement on the issue we're talking about.
I checked Dewitt and he has chapters on the Canon of Truth, and its relationship with Reason, which you probably ought to read first if you have not already. To me the best overview of the system before digging into particular details is always DeWitt.
But let us know how deeply you want to dig into that part because you may not find that necessary. I think the basics are the epistemology parts of the PD's (Bailey version):
- We must consider both the real purpose and all the evidence of direct perception, to which we always refer the conclusions of opinion; otherwise, all will be full of doubt and confusion.
- If you fight against all sensations, you will have no standard by which to judge even those of them which you say are false.
- If you reject any single sensation and fail to distinguish between the conclusion of opinion as to the appearance awaiting confirmation and that which is actually given by the sensation or feeling, or each intuitive apprehension of the mind, you will confound all other sensations as well with the same groundless opinion, so that you will reject every standard of judgment. And if among the mental images created by your opinion you affirm both that which awaits confirmation and that which does not, you will not escape error, since you will have preserved the whole cause of doubt in every judgment between what is right and what is wrong.
- If on each occasion, instead of referring your actions to the end of nature, you turn to some other nearer standard when you are making a choice or an avoidance, your actions will not be consistent with your principles.
Plus a pretty direct expansion of the basic physics: If the universe is matter and void, and infinite in extent and universal in time, then there is no "absolute" point of perspective from which someone can say that anything is "absolutely true." And there are no "ideal forms" against which to compare for absolute truth, and no supernatural "god" to ask either. There are only particular perceptions of particular things at particular times, generalized into summary opinions / concepts, and the "test of truth" is whether our opinions / concepts correspond faithfully to the thing we are observing at the time, as measured by the information we get from our five senses, feelings, and anticipations. And it's also important to remember that the senses / feelings / anticipations are our test for what is "real" to us at any time, regardless of whether we move to another level of generalization by calling them "true."
After thinking about this and glancing back at the Brown article, I think what I would recommend as much as anything else if you are interested in reading on this beyond DeWitt is the Appendix by Phillip De Lacey to his translation of Philodemus' "On Methods of Inference." The surviving part of the work by Philodemus is very interesting, but I would say start with the appendix on page 120. DeLacey gives a really interesting review of the development of Epicurean logic and how it relates to what came before in Greek thought. It has been a while since I read this but I remember that when I did, I thought it was excellent, especially in helping distinguish Epicurus from Plato and Aristotle.
I think we got started on this because Hiram pointed out that you probably meant "real" when you wrote "true," and that might be good enough for now. But the issue of reason and logic in the canon is pretty closely related to the same topic, and if I recall correctly the De Lacey article is a really good place to start reading if you want more.