Have you ever worked with someone who wrote for comics before turning to novels? Seeing how I write what could be called "fantasy thrillers" I've had little success in getting a short story published; most magazines and journals seem to favor literary fiction.
Not that I think that comics will be any easier to break into, but at least there are plenty of opportunities for writers like me who favor a more commercial kind of story.
I remember a case where we thought about representing someone who was really more of an artist than a writer. His prose was terrible, but the drawings were awesome. We told him to go write a graphic novel instead of trying to write a novel with illustrations.
If you've been published as a comic book writer, that's a legitimate previous publication and you should list it in your query letter in the credentials section. That said, you still have to write a great novel.