Caravan Water Concerns

I am a newbie to touring and took delivery of my 2015 Lunar Quasar in August. I have managed two weekends away so far, and both times have suffered from a bad stomach. One instance is still ongoing 2 weeks after the last trip away. I am keen to rule out the caravan water supply as a possible cause. I have flushed the system twice with Milton and clean water. I am concerned though because whenever I put water from the tap into a pan or sink, there is always a residue which feels like undissolved grains of salt. Has anyone else experienced this or know what it might be?