Bryan,

I've got an '88 Westy and don't camp that often really, but I just put plain
old city water in it and it seems to remain potable for a long time - as in
months and months.  We just use it for cleaning dishes and brewing coffee
for the most part though.  When my wife recently complained that the water
didn't taste all that great anymore, I washed out the big tank with just a
little plain old dish soap, rinsed well and refilled.  It seems fine now.
It could be that one of the differences between my situation and yours is
that our city water is chlorinated and my guess is that your well water
(while it's nice stuff) is not and therefore has a shorter 'shelf life' in
your tank.

John Graham
88 Westy - Charlotte NC