Really I think folks get too excited about cleaning out the black water tank. We use a "poop pellet" in the tank every time we use the RV, with the enzymes to break down the waste. Then when I get ready to dump the tanks at the storage place I have a little block of wood I use to hold open the toilet valve and I let the rest of our freshwater tank drain into the black water tank to fill it up.
Then I pull the valve and let 'er go! Follow up with dumping the grey tank and that's that.
I never bother with spraying out the black tank.
Steve
