At campgrounds we NEVER use direct hookup water.
We use only from the RV's fresh water tank and just refill it periodically from the hookup faucet. Doing this, I can always treat the water that comes into the fresh water tank two ways:
1. Dump 6 oz. of chlorine (for 40 gallons) into the refill hose before I screw it onto the hookup faucet so that the fresh water tank and whole system get sanitized with this chlorine every time I refill and eventually use - 40 gallons of water out of the fresh water tank.
2. One the other end of of the refill hose just before it goes into the RV's refill outlet, I screw on one of the blue Camco filters shown in the links provided by other posters above.
(I usually do 1., above, only at home before each trip ... but could do it on trips whenever we refill with water.)
In addition to these two things ... we NEVER drink from the the RV's fresh water tank on RV trips. We drink bottled water, only, that we bring along on each RV trip.