Add 9 ounces of Happy Camper Extreme Cleaner, fill the tank completely with water, let it sit overnight, and empty. This will completely break down and liquefy anything in the tank.
A clogged outlet is another issue and there are a couple of ways I found to address it. Close the black tank drain valve then raise the sewer hose as high as you can and have someone else fill it with water. Have the other person reopen the black tank drain valve and quickly place the sewer hose back in the sewer drain hole. This should create a suction action that loosens the clog. Another way is back flushing. If you have a back flush system try several flush and empty cycles until it is unclogged. If you don't have a back flush system you can buy and connect a back flush valve to your tank drain and back flush the tank with as much water pressure as possible. You can purchase these valves from most RV parts stores.
For black tank maintenance I use just one ounce (about 70 cents) of Happy Camper Holding Tank Treatment dissolved in a couple of gallons of water poured down the toilet into an empty tank. Then we just use only the amount of water you normally would for flushing the toilet and using the bathroom sink (which flows to the black tank in many rigs). No extra water is necessary as Happy Camper liquefies waste, including breaking down toilet paper. Dump when at least half full. This will keep your black water tank working properly and there will be no odor.
Let the inevitable flames begin.