The rheumotolgist is the right Dr to determine if you have RA, Lupus, both, neither, or something else altogether. It sounds like your doctors have waited too long to send you to the rheumy when so many of your symptoms point toward and auto-immune disease which they are not the best qualified to deal with.
As for your question, what does RA have to do with Lupus -- nothing. Of course, some people may have an overlap but I've not yet seen anything (since 2002) indicating that either influences the possibility of the other.
Take note of all your symptoms, when they start, when they stop, and everything in between. Lupus is diagnosed by having 4 of 11 symptoms, there is NOT a single test that confirms or denies Lupus. RA, on the other hand, I'm pretty sure there is a definitive blood test.
I would go ahead and take the cortiosteroids (there is no reason to suffer if you don't have to) but make sure you note all your conditions as they relate to what you are doing with meds, with sleep, with food, with exercise, with sun exposure, in general, note the general circumstances of life when the symptoms arise/exist. The more info you have the better job the rheumy can do.
Dunno where you are, but try to see the rheumy sooner than a few months!
Lynnwood, Lupus & Sjogren's Moderator
"Life is far too important to be taken seriously" - Oscar Wilde