waste water holding tank(s) inspection

I know it might be messy, but could you run your snake up the drain line? I fulltime and have had enough blockage due to slow running water and evidently food particles actually built up at the tank where it entered the drain pipe. I ran a small plumbers snake up the drain line and boy once it penetrated the blockage a little bit, the water/muck started flowing big time. I was catching in a five gallon bucket and the bucket was over half full by the time I got the snake pulled back out and the valve closed again to be able to hook up the slinky again.
You did say that your valve probably isn't working so it could be a big mess until you could get the hose put back on.