I'm interested in this topic because I'd like to be off-grid with my planned bus conversion, and this seems like a good way to get some water for laundry and watering plants. I looked up Oklahoma laws regarding rainwater collection, and not only is it legal, but residents are encouraged to harvest rainwater. OSU has published a document detailing how to plan how big a cistern you will need, various parts, etc. You can access a downloadable pdf here
However, I haven't seen any discussion of harvesting from the roof of a bus! Has anyone tried to do this? How? Is this practical, or a waste of space and effort?