More useful for boondockers who are spending a lot of time in the wilderness, if you can find a natural source of water to top up your reserves you can spend far more time out in nature before you need to head back in.

Making water drinking water has two components; filtration and purification. Filtration means removing particulate matter such as mud, sand, large plant cells such as algae and phytoplankton. Purification means just killing the germs and bacteria that can be common in natural water sources that can make you sick. Many methods will do both, but not all.

Not all water needs to be both filtered and purified. Some natural water from fast-flowing streams or springs may have very little particulate matter in it and be very clear already, and will only need purification if you're going to drink or cook with it.

See this guide from the National Park Service about how to treat water. Also see REI's guide that wants to sell you stuff (But is still useful).

Boiling water can be effective for medium quantities of water, at the cost of a lot of fuel used and potentially heating up the van significantly. Doesn't provide any filtration.

UV Sterilizer lights for your water tank van be a good safety measure, but shouldn't be relied upon as your only means of purification. Think of it as a good backup. Doesn't provide any filtration, and can consume a lot of power over time. Battery-operated ones are only effective for small bottles at a time.

Backpacking purifiers can provide both filtering and purification.