There are a couple of youtube videos showing filters applied to gray water and it being recycled for the toilet and showers.
It would be pretty easy.
On our water collection tanks at home I have two big blue filters, 25 micron down to one micron(I can go point two micron, but I don't) and a large uv light.
Point two micron water is hospital grade pure.
Less than $300 and a half day of routing water lines and you'll have almost no worries of running out of water.