Lee, I had the very same problem myself and asked similar questions on here. Its not an easy one.
Problem with filling up with tap water and then trying to filter it at point of use other than with di is you need much more water maybe 3 times more than what actually ends up filtered.
A merlin van fitted maybe a good option if he could fill up at houses as he's working?
I live in a first floor flat, I solved my problem by fitting a 240 ltr wheelie bin in a reduntant airing cupboard space, I use a merlin to filter, fills in just over 2 hours. I have a sub pump feeding a 1/2 " hose down to my front door where it plugs into a reel to my van, then takes 30 mins to transfer to a 250 ltr tank in van, I fill van of an evening and refill wheelie bin so I always have a reserve of water and never run out, not perfect situation, but hey it works! and was the only solution in my situation.
Hope some of this advice helps, theres usually a way round things! maybe others on here will have some more ideas
Simon