In a mad rush trying to catch up with work,after getting behind this week with the weather, I ran out of water.

This time it wasn't so much of a problem as I was pretty close to home - but still far enough away to be a real inconvenience.
So - I was thinking.
Would it be possible to have a Di tank as a back up for when I ( or if) I run out of my Ro'd water.?
So, if I run dry I can quickly connect a Di vessel, fill up with tap water - and away we go again.
I don't expect to run dry very othen, and wouldn't need to filter a huge amount of water thought the Di.
So, what size Di would be best?
Is it just a case of Mains Tap - to Hose Pipe - through Di Vessel - into Vehicle Tank?
Thanks