I found this guys web page very helpful when we planned our van. We used the same Shurflo bath and shower faucet and it is awesome and very controllable.
We have a 20 gal water tank which I find adequate. I rigged up a simple siphon system where I can fill a portable water jug at a camp faucet, set it on the van floor, put the siphon hose into the jug, flip a couple valve levers and turn on my electric water pump to pump it into the freshwater tank. I can add 5-10 gallons in a few minutes with minimal hassle. Water is heavy and takes up a lot of valuable interior storage space. I donít find the need for a huge freshwater tank with my siphon system.