Unless you find a forum that's more appropriate for your needs, absolutely use SDN all you want! Everyone gets protective and sometimes pretentious about their careers, so don't mind people who won't talk to you if you're not a doctor. You may not get helpful answers for something solely limited to RDH stuff (like questions about your exam or licensure), simply because we won't have the information, but if you bring up a concern about perio technique, office management, or patient issues everyone will be happy to discuss them.
The admins have a point about limiting actual forums to doctoral degrees (simply because they have to draw the line somewhere), so you'll probably have to deal with the dental forums and just make your own threads. Welcome!