Your body is dealing with a whole lot of new and frankly pretty terrible stuff when you have a chronic illness. I, personally, think it is more important to keep attuned to what your body wants and needs when it is broken like this, rather than forcing yourself to be just like you were before.
The weight gain may be related to the meds, it may be related to the inflammation in your body, it may even be due to stress or another autoimmune condition which hasn't been dealt with yet.
Prednisone definitely causes weight gain and weight redistribution. I've not been on that, but I am fat and I've been fat my whole life, even when I was working a job which had me walking the length and breadth of Manhattan my weight stayed constant.
If going to the gym makes you feel good, makes you proud and excited that your joints are cooperating and you aren't fatigued that's an amazing thing! That's something to be celebrated! But if you are fighting fatigue and joint pain to force yourself to work out because you want to lose weight.... That's less positive.
This may seem like a radical viewpoint (I'm sure some doctors would think so) but people can be happy and as healthy as possible for themselves at any size. Significant weight loss is difficult, and fewer than 5% of people keep that weight off for over a year or two. Take weight watchers, for example, touted as a super successful program. They count anyone who loses 10% of their body weight (so 20lbs on a 200lb person, clinically but likely not visually significant) who keeps the weight off for a matter of months to be a success. And let's say someone reaches their goal weight on weight watchers, regains that and more over three years, comes back and loses it again? That person is counted as a success twice!
It's hard to think that significant weight loss can be well nigh impossible, because we are so conditioned to see fat people as lazy and bad and out of control, and if we accept that maybe it's just a mostly genetic body type, like autoimmune conditions are mostly a genetic happenstance, we as a society have to address the fact we've been demonizing people based purely on their appearance.
You have an autoimmune condition. Your body is so much more complex that energy in/energy out (everyone's is, but especially chronically ill people.) Keep going to the gym and making yourself feel stronger and better, but try if you can not to make weight loss goal. Eat when you are hungry, eat what satisfies you and makes you feel good, move because it is pleasant to do so.
Though the title and some of the tenets are inherently flawed, you might want to check out Health at Every Size. It's written by a doctor who had been studying the negative terrible doomsday properties of obesity and found . . . It isn't actually bad.
We have so much to deal with as rheumatoid arthritis patients. Getting stressed over weight gain, distancing ourselves from our bodies even more than we already are by pain or dysfunction, will just exhaust us further mentally and physically.