Kendra, I am soooo so sorry.
I know how frustrated you've been with Harley's issues and I so understand trying to get down to the bottom of something and not really understanding much of anything. It's terrible when you don't know what exactly is wrong with your baby! Poor Harley has been through so much - thank God he has you though!
I have never personally used Revolution but Frontline, K9 Advantix and Iverhart have never upset his tummy.
My vet had mentioned something to me about a possible chicken allergy after Jackson's stomach problems and I am wondering if it's true. He was eating Organix which he was doing great on except his sudden really runny eyes.... and it did contain a few chicken ingredients. Well, I decided three days ago to start something REALLY simple, with venison as the meat source and I swear, it was almost immediate change. I picked up a bag of California Natural grain-free venison and the eye goop has almost gone completely away... don't know if it was coincidence or whatnot, but it seems to be helping, so I am gonna keep him on the food for a whole bag and see how it goes.... Now of course, Jax did not have the problems of bloody diarrhea, throw up, etc, when on a food w/ chicken, simply runny eyes... but yes, I would try to find a really limited ingredient diet without chicken as the meat source and take it from there!