I'm also on Humira and have gained some weight. However, I was on Enbrel for two years before which made me lose weight to such a point that I looked anorexic (and I ate like a teenage boy), so I'm okay with that little bit of extra padding.

A couple of thoughts. First, if working out this training you of all your energy, you are doing too much. The trick of living with this disease is to work within your limits and that means you need to stop while you still have energy left - this enables you to do things again the next day instead of having to recuperate for days. You will get less of a workout for a while, but sometimes when you have a chronic illness, you have to start with a minimal workout and very, very gradually move up. You may want to have a chat with you rheumatologist about referring it to a physiotherapist, who may be able to give you pointers on a workout that will protect your joints and your energy levels.

Second, not too long ago, we posted about diet and exercise for people who are living with RA and you may find some tips in those posts that can help you.  Good luck!