Ok, you have pulled the valve on your black water holding tank and you are either getting a tiny trickle or nothing at all. There are a few things you can try before you panic. 1st, you could try dumping 2-3 gallons of almost boiling water along with 1 cup fabric softener (or water softener) through the toilet, give it 15 minutes and see if that gets things moving. If you have a toilet wand, try pushing some water through - but this typically works when your toilet valve goes straight down into the tank.
2nd thing you could try is a plumbing snake. In this case, you want a piece of sewer hose that you don't mind sacrificing. Hook it up like normal, poke or drill a hole in the sewer hose, open the valve and try snaking through that hole in the sewer hose.
Best thing to do would be to have a Flush King or Hydroflush (or something similar) and hook it up to back flush your tank. This hooks directly to the camper; you hook your garden hose (not your drinking water hose) to the hose inlet; turn the water on and let the force of water "push" the blockage and break it up. Turn off and try to drain, turn on and off, on and off. It may take several tries for success! ALWAYS have someone inside keeping an eye on your toilet bowl. If your tank is full you run the risk of water running back out the toilet.