Is there a good medical care and treatment forum?

You know, the best thing is to take advice only from the medical personnel, trained, who know you and your individual case. There certainly are forums for folks with every sort of thing known to man, Parkinson's, Lewy's, Chronic Fatigue, you name it. You will there meet fellow sufferers. I once had a lot of IBS problems and walk helped by knowing what worked for fellow suffers; in fact found a medication recommend I asked my doc about, and tried, and was so effective against cramping that I began to fear "attacks" much less, and I believe it helped me put an end to a chronic condition I had for years.
So I am saying that you can get good help from fellow "sufferers" but you have to recognize that these folks are not trained medical people, often come up with supplements and vitamins that are pure nonsense and expensive at best, harmful at worst.
So this is a case of decide for yourself and take care. Type in the condition you want to follow, followed by Forum, and go for it. For instance "IBS Forum".

I don't have a very common condition, so I doubt there are any national sites for it.

If you are having a specific medical issue I'd google to find national websites for those illnesses/diseases.