Man, what a crappy situation to be in! (Come on, you've all been waiting to say that).
I don't think ice cubes is going to work. A video on a well known rv web site debunked that myth. What you need, depending on what is causing the clog, physical force to break it up. If it's is a piece of plastic pipe as some suggested, you might have to cut out and replace the piece that's clogged. If it is just waste, you might want to try the broom handle if you can get a straight shot or use a wand with a spray head.
Good luck.