I would suggest RC sensitive too, and other high protein, non processed meats (though a raw diet can be difficult to introduce).
Slightly different, but when we got Tess ( our last dog) she had dreadful digestive issues due to having been fed an incomprehensibly unhealthy diet from being weaned. It took us two years to get it sorted, with various types of food (ultimately Chappie proving to be the best food for her - not appropriate for cats though).
It can be so difficult to pinpoint food intolerances, but if parasitic infections have been ruled out, then it seems to be the most likely cause.