Ok, not a doctor, but this is based on my experience of being little with IBS and GERD and what my doctors did...
Well, it sounds like typical gastro stuff to awhile... and then it sounds bad. When I first got sick my pediatrician did EXACTLY what that gastro did, which is fine for a pediatrician. When the pepcid (which was prescription only at the time) didn't help, she referred me to a pediatric gastroenterologist. Who ran a bunch of tests on me and then told me that she could run a bunch more tests on me or control the symptoms of my probable GERD (which is what pepcid is for, not IBS). I didn't want any more tests so I got medicine. The tests are hard to do on children, because their digestive system hasn't finished growning and is quite small, but you can do them. (You might have to do them again as an adult). At the very least they should have done an ultrasound and checked for organ damage (if I'm remembering correctly) and a blood test or two. I didn't get diagnosed with IBS too until a few years later down the line after I'd switched medicines quite a bit and some of my symptoms were fixed and some just wouldn't go away.
I would suggest taking her to a new gastro who will do some tests on her. I always suggest women doctors. I know it's sexist, but they have just repeatedly worked out well for me (my pediatric gastro and my current gastro are both women, the 2 men I tried going to inbetween sucked).
Arrgh, hang in there. I hope your daughter appreciates all the love and support you are giving her though. Sometimes I think getting those things so early is a blessing. People around here find it hard to adjust their diet and way of living when they get it as grown-ups, but I have always eaten for my GERD and IBS and I'm a healthier happier person for doing so.