Hi there. I am an author with a couple of self-published works. For me, RPing and stuff actually fuels my books. There are times, however, when I would really rather work on my own characters. When November and August roll around, I put up a hiatus notice or I tell them in advance that my activity will be relatively slow. (For NaNoWriMo and Camp NaNoWriMo, respectivly).
Most of the people I RP with are also authors if not slightly more on the hobby side than the publishing side. That might be a route you can go, though. Let them know up front when you just have to write.
If that fails, set a specific time for JUST WRITING. And they should respect it.