I carry 25 gallons (for two adults and two dogs) and consider it enough water. It lasts a full week for us (and one of our dogs is part camel). I find water easy to come by and do not feel the need to carry more from difficulty or scarcity. I would consider 40+ gallons to be a ridiculous amount to have in a van. It is a massive waste of space for a resource you use only a tiny fraction each day. You will be unable to carry any food with all the storage space taken up by water, meaning you will have to go right back into town for food more often, eliminating the whole purpose of carrying so much water.
Part of van-living is living within one's means. If you want to continue living in a van, you have to drop the sticks-n-bricks mentality that huge amounts of water (or electricity) is necessary to have a happy or clean life. If you must hold onto your previous water use from your sticks-n-bricks life, you will not last long in a van. 3 gallons a day usage is a huge amount for a single person. If you can't pull that off, I fear your plans are already doomed.
Instead, I offer the out of the box ideas that come standard to most van-dwellers...
1) bathe elsewhere (gym, community centers, lake/stream, etc)
2) bathe less often (it is a false that you must bathe daily to be clean)
2) bathe from a pot, pan, bowl, or bucket with a washcloth and soap and use only a fraction of the water
Good luck