If you use the forum search facility lots will come up, including how to blow or pump out the system.
For a more concise answer, do you have separate water cannisters under the sinks or one onboard tank and is any hot water system fitted?

Colin

The short answer will be empty the cannisters and run the taps dry. Next turn off the 12v and then turn open the taps and leave them open over winter. Some find putting a blown up balloon over the end of the tap useful as the air it releases does get some water out of the tube and pump.
The idea is to keep the van inside from freezing over winter as this can split a tap body possibly costing £100 plus. A new pump is about £20.
Don't forget to keep all cupboards open to allow air circulation and if you can get upholstery indoors.

Colin

I forgot to mention the loo, so we'll done that man. As regards taps etc, I do as you do, but I have an onboard tank and water boiler.

Colin

Some are not as hardy as you. I'm a wimp. March to October normally offers decent enough weather to not get soaked and frozen at the same time.
I wonder if those of us with larger vans tend to stretch the season more as we have greater space to relax indoors. I also think hard standing pitches make the season longer.

Colin