This isn't an unusual occurrance. Campgrounds should be doing a testing of their water systems on a periodic basis. Having managed a group of 25+ public lands campgrounds, it was not unusual to get one or two "bad well" test results occasionally during the summer. We would post the well as unpotable and then shock the system with chlorine, followed by further testing. A clean bill of health would usually be recieved within a few days. The bad reading could be caused by many things. (usually a higher than acceptable total coliform bacteria count - this does NOT necessarily indicate E. coli bacteria is present). It also doesn't necessarily mean that the water is undrinkable, but may impact people with compromised immune systems, the very young, elderly, etc. A wise person would NOT drink the water, in my opinion.
Here is a typical report summarizing a season (from 10 years ago) for public water systems and what was done.
2006 Water Monitoring Report As for your own FW tank and water lines, I would simply do as others have said. Just sanitize your trailer's FW system as you normally do at the beginning of your camping season. Then you should be good to go.
