Buy the OP's description it sounds like they have cable operated drain valves. Again from the description it sounds like the OP might have a poo pile. In other words they did not use enough water in the black tank or attempted to drain it before it was full. If they have enclosed underbelly then he needs to remove the plastic panels and insulation to access the valves. Then it should be a pretty simple 4 bolt pattern to separate the valve and slide the blade out for replacement. But, before I went to all that trouble, I would fill the black tank with water and after letting it sit for a while open the valve and see h=if it drains.
