When I was a kid growing up, we had a beagle that had terrible allergies in St. Louis. My parents were teachers and we spent the summers in northern Minnesota where my dad was a director in a summer camp. Our dog had no allergies when up north, but as soon as we got back to St. Louis in August, he was miserable until the first frost.
Flea allergies, bad ones, are among the worst skin conditions we see in dogs. Like you said, they are truly miserable. And they can do some serious self mutilation because of the itching.
Don't be too afraid of the steroids, they can really knock out the symptoms quickly (better than antihistamines) and allow time to get the flea situation under control.
Hope your plan works quickly.
Doug