I had the same problem after my trailer sat all summer, nice green crud. The problem I had was getting all the water (and algae) out of the fresh water tank. I got under and would push around on the bottom of the tank and try to get the water over to the drain, but then the problem was with the drain. The fitting stands up proud of the tank wall, so I never could get it all drained.
I plotted how to get the tank free so I could tilt it or even how to make a different drain hole. Eventually I just made sure I bleached it good and rinsed good, filled it up and hit the road. We don't drink it, and I never noticed it come out the faucet, and it was all gone that I could tell when we got home.
I haven't come up with a good way to drain the fresh tank completely.