Your water bill is made up of roughly 95% of which go's down your plug hole toilet, bath, shower, dishwasher etc. That is returned to sewer, that is charged at a higher rate roughly £1.30 per cubic meter (1000 lts). None return to sewer that is the water that dose not go down your plug hole, Drinking, cooking, watering the garden etc that is charged at a lower rate as it's not returned to the sewer so they dont have to treat the water to use the water again. So as you take all your water away and dont put it down the drain you can claim the lower rate, providing you have applied for a charge of none return to sewer. So nip down to you local plumbing merchant and get a meter, (£40.00) then fix it to your system or your supply that feed's your van and that is your sub meter.