I don't suppose it would fit under the house on the downhill side, without having to do all that excavation? That would also make it easier to drain for the winter and avoid needing a sump pump to keep it from floating out of the hole.
Great idea and I sure wish I had thought of this before we built the house. As it is we have a bit over 4' clear under the house on the low side. This tank won't fit, but maybe I could find a low profile plastic tank that would fit. The crawl space has a pretty good cross slope to it so we would need to invest some effort in trying to level it. This will probably require some rock removal. Wonder if maybe I could find a surplus water bladder? Boy, that might work out really well.