Not sure how much water you have in the black tank, but if you have water in it, you might want to try to use an enzyme type chemical to where the enzymes will eat up the solids. But this will take some time and you will have to move out of the trailer if it smells bad. Pour some Happy Camper in it and let it work for a couple of weeks, then open the black tank gate valve and it should drain, but make sure there is plenty of water in it. If there is no water, Then you have big problems and the best thing would be to call a plumber.