Depends on where you fill it up. Be careful with filling up at sites. People might (mistakenly) use the provided hose to rinse their portapotti's / chemical toilets.
We fill the tank at home, using the garden hose. There's a filler cap you can use to connect the hose to the tank's opening. At sites we usually fill it with a 1.5L plastic bottle. Which works surprisingly well. A few bottles and we're fine for quite a while. But we use the water mainly for brushing our teeth, washing hands, making tea. etc.
To keep it clean, we add a full tube of steradent tablets. (The ones you would use to clean your false teeth). Drop 'm in the tank and add water untill its 3/4th full. Keep 'm there for 24h and drive around a bit. Drain the tank after that. Water tastes like mint for a short while. This is not something I conjured up myself. It was tested by NL's biggest camper club and proven to work well.
There's also solutions where you hang a big bag of water from the van and fill the tank that way, but it always seemed like a hassle to me.