MCTD includes lupus which can attack your joints which it did to me...all of them with incredible swelling and pain. Can't give you any advice about fibro as I have never had it.
If you have barretts which I also have I would doubt that gluten has anything to do with it. Barretts is caused by acid reflux and is often associated with mctd and/or myositis diseases. The round muscles at the top of your stomach are weakened causing the acid reflux. If you have barretts then you know that is a precancerous condition and you should be taking a PPI to reduce the acid and get an annual endoscopy exam.
I don't think that typical ER docs are going to be able to help you. find another rheumy. 10mg of pred is a low maintenance dose, not what you would prescribe to someone with an active disease. I was on 60 mg pred and my case was resistant. It took a massive 5 day treatment of ivig to save my life.
Most cases are not as extreme as mine was but mctd is very complicated and can be difficult to treat just like lupus alone. It is lupus plus 2 other diseases plus whatever else you might have.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 25mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.