My perspective is that many campgrounds have city water supplies and if they are on a well system, the water supply is usually inspected and certified. I suspect that it is certainly in the best interest of campground operators not to kill off their clientele. Just think of the liability! Additionally, I rather doubt that tent campers filter their water or worry about taking a shower in the bath house.
My memory is a bit foggy
but it seems to me that whole house water filters in trailers only appeared about ten or fifteen years ago. My belief is that the first RV manufacturer to install them did so almost as an inexpensive (to them) sales gimmick. But, it sounded good and pretty soon all the other manufacturers were forced to do the same. However, it has never made much sense to me to filter the water that I am going to use to take a shower or flush the toilet. On the other hand, I do agree that it might be wise to filter your drinking and cooking water if you are not hooked up to a known water source.
To answer your question about “How do I get a pretty good filter for decently cheap?”
May I suggest you go to the following site as they have a lot of good information that will enable you make a good decision. http://www.rvwaterfilterstore.com
“Second part of question is how to know whether your water will be good or not? Examples, will it just plain ol be unsafe , full of sand, etc? How do you go about doing this?” The simple answer is Ask. Check the campground reviews on line. Ask when you check in. Ask other campers; they will be only too glad to tell you if there is a problem.