Any difficulty breathing, voice changes (bark sounds different), etc? Laryngeal paralysis is common in older dogs but would have some obvious signs for her to get to that high of a temp (from being unable to breathe well). Yes, cooling hopefully has been initiated - usually we want to get them to around 103F before stopping cooling measures (so they don't continue to go down - they can be cooled too much).
Cancer would be another possibility - usually need abdominal ultrasound but I've not had any cancers cause fever that high. Severe infection, abscess (sometimes internal ones), sometimes it's a fever of unknown origin and that's when the steroids come into play. If she had a severe abscess/infection the CBC would likely show a veryy high WBC (no steroids in that case!). Anti-inflammatories should be used cautiously with steroids (or not at all, or with stomach protectants). Any chance she's a Shar-Pei mix? Shar Pei fever sometimes can run that high, though usually not quite that high, more around the 103-104 range. I'll assume heat stroke has already been ruled out - although a dog with laryngeal paralysis (lar par) being outside in these temps and humidity could definitely get heat stroke.
Good luck - I'm sure the ER vet is still very much involved with the case even though they are busy. Hope you get a diagnosis soon.
