blocked black & grey water tanks

You may be able to siphon or pump most of the liquid out through the toilet. At least get enough out that when you drive it there is room for some sloshing around of contents. If that doesn't work you are going to need a really good friend to help you. One hold the slinky and the other on feed a snake up the drain line (if I was the friend you would be holding the slinky). Poke a hole through whatever is blocking the tank opening. Once it is open retract the snake and get the slinky hooked up. As the water flows it should open up a larger hole. After the tank is drained get back in there with the snake and work it through the opening of the tank several times to bust everything up. Fill and flush to get the rest cleaned out.