I have a water meter and wouldn't dare to fill my van up at home!!
I Use a merlin ro with a 350 litre tank, & 1 Di bottle and fill up as I go,
Good filters gives me between 15 & 25 tds after merlin and will cost you nothing.
Use commercial sites if poss as long as your waste goes down a grid and don't make a mess, they won't bat an eye lid.
£350 a month is mega bucks down the drain,
I will fill ya tank for ya every day for less.