Although I think its impractical to use inside a container the principal would be good to filter gray water for plants and washing.
Like to incorporate some water storage tanks for drinking and cooking some where inside the furniture or maybe the floor itself would also insulate it?
Water bladder type storage would be idea in the wall or ceiling of the bathroom, I like idea of ceiling but realize some reinforcing will be needed, but could gather rain water from roof of container and solar ledge to the bladder used for natural force shower and washing then filtered for re use in aquaponic system?
Some water facts!Water weighs 1kg per Litre
1000ltr tank = 1 ton in weight.
1mm of rain per SQ Meter = 1Ltr of water
Not forgetting some water filters on all kitchen uses, the Caravan/ RV and boat market will be a good place to research this need.