There are basically two types of water filters, sediment and contaminant.
Sediment filters will filter out the little specs of sand, dirt, rust and other similar particles. From what i have seen, these are in the 3-5 micron filtration range. This type of filter usually does not restrict the water flow very much.
Contaminant filters will filter out such things as arsenic, bacteria, lead, mercury and other contaminants. These types of filters have a 0.5-1 micron filtration range and may restrict water flow somewhat more than the sediment filters.
Depending on the brand, either type of filter may also reduce the smell, taste or amount of any chlorine in the water. Some brands, such as many RV related brands, combine both functions into one filter.
Neither type of these filters will purify the water! This would require special treatment, which I won’t get into here, but these types of filters will not make unsafe water safe to drink.
At a minimum, I would suggest some sort of sediment filter to catch any dirt or sand that might find its way into your water hoses, or to catch any rust that might be in the water pipes from the campground or its water supplier.
The contaminant filter is a bit different.
If you use the campground water supply for doing dishes or taking a shower, you probably don’t need a contaminant filter.
If you use bottled water for drinking, cooking or brushing your teeth, you probably don’t need a contaminant filter.
If you use the campground water supply for drinking, cooking or brushing your teeth, you may want to have a contaminant filter because…you may not know where the water is coming from (of course you can always ask).
Some campgrounds get their water from a nearby city or town which is usually treated and safe to drink, while other campgrounds may get their water from a well, a lake or a river nearby and treat the water themselves. Regardless of where the water comes from, things can happen in the treatment process that could allow contaminants to pass through the system and get to your water faucet. (Ever hear on the news where a city or town is urging residents to boil their water because something went wrong?) However, it is more likely that problems will crop up in the water system of a campground that gets its water from a well, lake or river simply because they may not monitor it as well as a bigger city or town would, or something out of the ordinary gets into their water supply.
Many of the newer RV's have a separate water spout on the kitchen sink that is connected to an in-line water filter, usually a contaminant filter, to be used for drinking water. If you have one of these water spouts in your RV, check out the filter, if there is one. This could cut down the need for a separate contaminant filter on the outside.
If you use a separate sediment filter and a separate contaminant filter, put the sediment filter first in line.
As with the water pressure regulators, some RV related water filters will reduce water flow. Check out the whole-household water filter systems available at home improvement stores (like Home Depot or Lowes), a good plumbing supply store or even Sears or Wal-mart. These filters or systems will cost a little less than the RV related brands, they should be easier to find replacement cartridges for while on the road and most will do just as good of a job as the RV related brands if not better. Because you will be attaching these to your water hose, don’t forget to get the extra fittings you will need to hook them up.
Now, where do you put the water filter...before or after the pressure regulator? I would suggest putting the filter before the pressure regulator. That way there is more pressure to push the water through the filters and maintain a good flow of water through the rest of the system. I use a short 4' section of hose from the campground water faucet to the filters and straight to the pressure regulator, then I have the rest of the hose to get to the RV.
Read the instructions carefully, paying particular attention as to how often you will need to change them and how to store them. (Since most household filters would be constantly “wet”, you may need to store them that way. Make sure you have a way to do this, such as a zip-lock baggie or hose end-cap to seal them off at the connections.)
Okay, I'm not a water filter expert. These are just some of the things I have learned about water filters and water systems. Most major water filter manufacturers have websites you can check out to get more information than what might be on their packaging. Do some research on your own and make your own decision on what type of water filter system is best for your needs.