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I have always used tap water.
I have never lived where the water is hard.
Just west coast tap water.
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The west coast is a big place... our tap water is very hard, as is most of the water you'd get in the 7 or 8 southwestern states.
I carry two gallons of distilled water (less than $2 almost anywhere), and two gallons of undiluted antifreeze. It seems these days that just about all of the labels have "compatible with all engines" and a number of other non-specific claims, but I've found the cost between the "cheap" stuff and the "good" stuff to be negligible. 
I happen to have a gallon of diluted in the van right now, just because. I also carry "puke buckets" because my kids get carsick from time to time, so along with some other odds and ends I could replace my coolant on the fly in most cases, and if not I'd have two gallons of emergency drinking water if I needed it for that.
I consider that a good trade-off in terms of weight/space, so I don't use the pre-diluted stuff.
I was just telling somebody that when I was swapping out my alternator recently I bumped a small coolant line that lies under and to the right of the alt and the thing just cracked and crumbled. I just recently completed an almost 1K-mile trip; I'm amazed that the thing just didn't blow off. That's both lines from the heads to the bleeder ring that've gone bad in recent months, so I'm gonna check the rest of the ring and the other small hoses. I guess that's just another thing to add to the two-year maintenance cycle.

Cya,
Robert