First thing I would try is straight white vinegar, spray it on, let it soak for a while and then try scrubbing it off. Repeat a few times. If that doesn't work, I would check out this page about cleaning hard water stains from windows, many of these approaches may also work for you.
http://www.thriftyfun.com/tf351538.tip.html
If no one here can help, you might ask this question at the Mobile Home Repair forum. Just Google "mobile home repair forum" and create a free account. Those folks have helped me with a lot of issues in maintaining our double-wide, and I'm sure they can help you, too.
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