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A lot of folks use 4" or so PVC in their van or on their rack for water storage. Schedule 40 is safe for potable water; some other PVC pipe may not be because of added plasticizers.
If on the rack and exposed to the sun, scruff it up with an abrasive pad in prep for painting, then paint it flat black so it warms up nice for showers. Some even pressurize the pipe so they have water pressure.
A word on water containers: Even military water jugs are not meant for long term water storage. Contents should rotated out; used regularly and replaced with fresh.
Whatever you store water in, especially when on the road long term or way back country, sanitize it before using and from time to time afterward with a light bleach solution. No fun to be away from population and have stomach or intestinal problems from bacteria in your water supply.
From the US Navy: "To sanitize containers and utensils, mix 1 tablespoon Regular Clorox Bleach with one gallon of water. Always wash and rinse items first, then let each item soak in Clorox Bleach Sanitizing Solution for 2 minutes. Drain and air dry."
More in the attached pdf from cnic.navy.mil on emergency water purification, storage, disinfection, etc.
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