Not sure if this matches your symptoms, but it is a common problem with iron in the water in this area. A water softener will usually take care of it.
I'd guess that if this is your problem, the issue is caused by increased usage since converting your camp, or being there at different times of the year.
http://dnr.wi.gov/org/water/dwg/febact.htm
It also can help to bleach your well on occasion.
http://www.deq.state.ok.us/csdnew/disiwa.htm
But as suggested, I'd start with a water test.