For me the answer is deciding how much time I want to put into either activity. I agree that RP can feel draining - at times it can be like a black hole that consumes your creative urges, but in a nice way - and in order to be able to write both for fun and for, well, work, I try to separate them. Writing for my book, stories, etc, is work. I decide how much time I want to put into that each day, and then try to delete as many disturbances as possible - I write with a pen and paper, am not logged into chat and won't check my emails. It's hard, because when I RP I do a hundred things at the same time, I write, I chat with people, I fiddle in Photoshop, I google pictures of my characters; but when I write other texts, I try to focus on that alone, channelling what creative force I have that day, into that text alone. (Goes without saying there are days when I hardly write anything, but I consider that hour of staring out the window a small victory anyway, because I stuck with it.)
So, my advice would be to decide how much time you want to spend with RP each day, and how much time you want to (try and) write. Tell your RP partner why you're doing this (because seriously, we should respect each others dreams!), and how much time you can spend with him/her. Then try to get into a habit where your novel writing and your RP writing each have a spot in your day, but not at the same time, and not more RP than novel writing.
For me, that focus was what I needed to release energy - creative or otherwise - and start using it for novel writing. Whole evenings stopped disappearing to the art of RP, and I began writing other things with more ease.
If you want to discuss this further (in the time you're not writing of course :P), just send me a PM, I was really happy to see that there was another roleplayer who had the same thoughts and problems as I did! <3