I'm pretty new as well, and have had to deal with more than a few problems. The people on this forum have explained a lot to me and have helped enormously.
My experience with the drain valves is limited to freezing and thawing. But if yours is not frozen due to low temps, my guess would be that it's mechanically stuck. In either case, I'm pretty sure you have to gain access to the actual valve mechanism to work on it, which on my unit means dropping part or all of the underbelly (the cardboard-like sheet that's underneath the unit). That's the only way I know to gain access to the valves on my unit.
When we were in Colorado for 5 weeks, the nearest dealer was 2 hours away with a tough drive thru a mountain pass - with snow on the roads most days. Getting a mobile repair guy in would have been pretty pricey because I'd have to pay for their travel time. All that to say that I was willing to get pretty adventurous in fixing things because I really didn't have any other good choices.
Anyway, if you can follow the large drain pipe back to where it joins the tank, 10 or 15 minutes with a hair dryer will probably get it working. If that doesn't free it up, and you can get to the valve, you might be able to open it with a pair of pliers.
If it's not working, but you can get it open, for a temporary workaround, you might be able to use one of these
http://www.amazon.com/Valterra-T58-...JS/ref=sr_1_13?ie=UTF8&qid=1298773719&sr=8-13
It goes on the end of the drain pipe where you'd normally attach the sewer hose. You'd be able to leave the broken black valve open until you can get it fixed, and not leak sewage in the meantime. If you did use this in cold temps, you'd probably have to put heat tape on the drain pipe to keep the sewage from freezing in the exposed drainpipe area.