Not all parks and stops have clean water suitable for drinking. Consuming such water can lead to sickness of varying degrees, with giardia a common result. A great way to prevent many nasties is to use a high quality water filter.
Water filters can be plumbed inline near your caravan's kitchen sink, so you have a special drinking water tap which is filtered. Alternatively, you can filter all of the water entering the van – however, due to the much larger volume of water being filtered, you will need to replace your filter cartridges more often. The benefit of filtering the water before entering your van is that you don't put the contaminants in the tank in the first place – helping your tank to remain fresher for longer.
Generally, you get what you pay for when it comes to filters. The more expensive ones use better technology and filter out more things. A dual filter system can be a cost effective way of getting the highest quality drinking water.