I imagine it will depend on how severe your RA is.
My friend/weight loss mentaur at work lost over 100 pounds a year or two ago and was doing a great job keeping it off. Last September she developed RA. It started in her wrist and spread throughout her body in no time. I gather that this was very unusual b/c her doctor said he rarely saw new RA sufferers in such acute distress.
They gave her an anti-inflammatory steroid, Prednisone, which controls the swelling and makes her feel better, but anyone who has ever been on the drug knows it tends to make you very hungry and makes you retain water. She told me she put on 30 to 40 pounds between September and January.
Like I said, I think her case was unusual with it being so severe, but I would think that if your doctor chooses to treat you with similar drugs, it may be a bit of a battle for you with weight loss. My friend has continued to exercise, though, when she can. She just has to be more careful not to overdo it now.