If you are only travelling sporadically, leaving water sitting inside a water filter or a filter housing can be a bad idea, as it makes bacterial growth much more likely.
Whenever the caravan is going to be left unused for several weeks, remove the water filters and leave them upside down in a sink so the water can drain out.
Once reasonably dry, store them in a dry location — preferably in a clean container so they can’t get dirty.
If you are using cannister-style housings, you should put the filters back into the dry sumps. This protects them from dust, dirt, and other contaminants.
Contrary to popular belief, water filters should NOT be stored in a refrigerator when not in use.
That’s for two reasons. The first is that the carbon will start to absorb the aromas of the food in the fridge — and you might not enjoy bacon flavoured water!
The second is that the filter will be more likely to develop bacteria due to moisture and any old food left inside the refrigerator.