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.
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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