I recommended using a filter every time you fill your RV's fresh water tank. We use an in-line filter that screws right onto the hose I use for filling the fresh water tank. By the way, always use only a potable water hose for filling of your RV's fresh water tank.
Also, never fill your fresh water tank from the water pipe source at a dump station. Many dump station areas are posted with a sign implying that the water there is not to be used for drinking (i.e. "non-potable water") - but some dump station areas don't post one way or the other - so I choose to not trust any dump station's water hose for filling of our RV's fresh water tank.
We also have a built-in filter in our RV's kitchen sink faucet inlet pipe. Even with all these precautions, for drinking water we always bring along bottled water for drinking. We never drink directly the water from our fresh water tank. We never buy "purifed " bottled water either - only bottled water sourced from a spring.
Before each trip I empty the RV's fresh water tank and completely refill it. Before the refill, I pour about 6 oz. of chlorine (we have a 39 gallon fresh water tank) into the end of the filling hose just before screwing the hose onto the house water faucet. This is enough chlorine to sanitize the RV's 39 gallon tank and fresh water system without causing a chlorine smell from the faucets or shower head.
Our RV's fresh water tank inlet has a locking cover over it, as an added precaution.