Water treatment once the van is out of storage

Hello,

As a newbie to caravanning im wondering if you can give me some advice about water treatment once i take my van out of storage.

It would have been sat for 4 months by the time we take it out next month for its first service and im just wondering what everyone does about treating/flushing out the water system?

Do you use a caravan branded water treatment to flush the system or can you use milton?

Or, do you just make sure the water is boiled before use?

Thanks in advance for your responses....just want to make sure i get everything right before our first trip away this year!