With a portable water system (such as an RV) it is a good idea to sanitze often. If I am testing a private well I sterilize the tap nearest the tank as I don't want to test the plumbing, just the water supply. Notice I mentioned not testing the plumbing. Microorganisms can and do hang up in the plumbing. If you hook to a questionable water supply it is a good idea to sanatize the entire system. Don't count on the check valve to protect the tank, it probably will but why take the chance when prevention is relatively easy.
This is not to say you will come down with Montazuma's Revenge or Cholera
from a non-potable system. Most of there are safe enough that your immune system will take care of minor problems. I am not recomending that anyone do that but it is probably not the end of the world (operative word is "probably"). There are health standards and aesthetic standards. High disolved iron 9indicates iron bacteria) will give a taste but iron bacteria are not a threat to the health of most people.
I recomend sanitizing the tank water and running the tank water through the entire system. Run each fixture for several gallons and work the valves from open to closed and back open while doing the sanitizing and the flushing. Flush twice. It is not a bad idea to sanitize before winterizing.
There is no filter I know of that is 100% effective in removing all microorganisms. There are some that come quite close, viruses are still a problem. These are all pressure systems that use R/O or UF/MF filters. Hypochlorite is the most effective sanitizer for the average person (Chlorox Bleach).
You can always invest in one of the personal water filters. They are available commercially but generally not worthwhile unless you do a lot of hiking and tend to forget your iodine tabs or really want to rough it. The amount you would spend on a good pressurized filter system is quite high compared to just periodically sanitizing the system. You still have to maintain those systems.
I am not trashing filters, I do have a Culligan internaly on my system and use a line filter on my fill up hose (when not using a familiar system). These are mainly general purpose filters to keep out suspended solids and tie up some disolved substances, for improvement of taste and clarity.
Most older water tanks will develop small amounts of black-blue/black colonies. These are often a combination of filimentous bacteria and mold. As a rule these are not harmful but do not contribute to asthetics. Traces of the colonies may remain even after sanitizing, even though the colony is dead. I do not concern myself about those in systems that are regularly sanitized. Most standard line filters will eliminate any small amounts that would come down the lines.
When I had my tank out this summer I hot rinsed it then put in a gallon of 7% hypochlorite before filling the tank. I let this site in the hot sun for eight full hours then dumped it and single rinsed the tank. I then added a cup of hypochlorite with a gallon of water swished it arround then emptied the system. As my system is emptied for the winter and the plumbing is being replaced I will not winterize. The two or three cups of remaining liquid will not hurt anything and it will maintain a high residual chlorine and choramines. I will have some trace chlorine in the tank atmosphere all winter while keeping the level low enough so as not to jeopardize any components.
Getting to the main point! SANITIZE regularly! Use filters for aesthetics not as your water safety net. Do not connect to questionable systems. carry a couple of five gallon water cans for wash water from such systems (add a couple tablespoons of Chlorox to each can and let sit for a few hours).