drain water out of tank with a hose pipe, you might have to suck it out, then either get it out of van if you can if not leave in, and pour a load of hot water in the top untill it is full, and leave for about 30 mins as this will melt some of the water, then drain again and repeat, it is pretty easy once you get hang of it, hope this helps, as this is what we use to do to tanks that had frozen