When you go for dehydration, have they found objective signs of dehydration, e.g. low blood pressure, increased heart rate or breathing? Did they give you IV fluids or any painkillers at the ER?
If they did, there's really not much more you can expect from them. They'll be able to recognise if your pain is due to a medical emergency, like a blockage or perforation. If your show no symptoms of something like that, it's not their job to diagnose the cause of pain. If you show no objective signs of dehydration, they can't treat for it (and there'd be no need to).
If you need to go to the ER sometimes because you're not sure if you're in an emergency situation or not, or just want reassurance that you're not, that's a valid reason for going. If you need strong pain relief, that's a valid reason for going, so take no notice of anyone there who suggests otherwise (as long as you're not going every week for these reasons - e.g. you can go to the ER once just for pain relief until you can see your specialist, but if you're needing stronger pain relief regularly, your specialist should be addressing that).
But ER doctors don't need to know about Crohn's, they just need to be able to recognise its emergency complications. When I went with a blocked bowel, the doctor I saw wasn't a specialist, he wasn't a gastroenterologist, but he could recognise that I had a bowel blockage, and then he got a specialist surgeon to come to see me - it didn't matter that the emergency doctor didn't know much about Crohn's himself.
Of course this still doesn't justify them treating you like you're just there for pain meds, but if your records show you go there regularly and request pain relief, when you're not in an emergency situation, I guess some might start to question it. When you go, do you specifically ask for pain relief, and do you request a specific medication, or do you wait for them to bring it up?