I'm on one unfortunatly. My bills are quite high. I use about 1600 litres a week so you can imagine how much waste water I'm wasting. I've got a few ideas though to try and stop this though. I have a few customers that have said I can fill up from their house. Only problem is that I would be hanging around for 4 hours whilst tank fills. The other idea would be to just fill my tank from their tap normally then transfer into another tank at my house via merlin then back into my van again. Time consuming yes but lower water bills.
Someone I know takes a bit of water from peoples water butts, puts it in his trolley, the TDS is usually about 25, so when mixed with your standard water it's ok, as long as you don't do too much!! Not sure exactly how he transfers it.
But rain water harvesting could be an option I'm pondering it...