For long term, vehicle based travel, the simple solution is to re-use the same water over and over again. This eliminates concerns with the source of your water, time and/or cost to find it, time to purify it, etc.
Sink and shower basins drain into a holding tank, but instead of being held for dumping (gray water), it is slowly filtered and dumped back into a fresh water tank. Systems using these principles have been around for decades and are used on a commercial scale the world over.
Once the water is run through the system, the only filtration required is what you put into it. This means filter the water once, use it to wash your hands and when it goes back into the gray water holding tank, it doesn't magically have heavy metals, giardia, cryptosporidium or polio in it. So, there is no concern over filtering these out.
Lifeline, Diercon and Sawyer all make filters to 0.1 micron. Pre-filters to remove food particles, dirt, hair, etc., before the finer filters are clogged up, would be required. Very little water is consumed using this type of system and 350L (92 US gallons) would provide water for months on the road, with only an occasional "topping off"
Never understood why people wanted to "consume" water, when it can easily be filtered and re-used over and again. This applies whether you are in a vehicle or a municipal waste water treatment facility. How many places in the US have water "shortages"? Do they treat their waste water and re-use it or dump into into a river or ocean? Filtration sources can even include seawater or urine from healthy human beings, which is about 90% water, with the rest being uric acid and salt.
http://worldhousingsolution.com/water-sewer-systems/
