With 2 kids and a wife I go through some water, between washing the kids off every time they have to go inside to pee (don't get me started on that) and washing everything off I go through the internal 25 gallon tank in about 1.5 days. I have an outside shower and use it to lower the grey tank usage but having a big tank on top for sure helps.
Matter fact I am on my way down there after I get off work, I will talk to some of the guys in the bullpen to get some ideas (since we will all be crowded together from what I hear), I might just go the 6" black PVC route so that will have the weight more distributed.