Originally Posted by cwf
I have a filter on the blue/white water supply. Also one under sink.
Since both filters use granular activated carbon (GAC) as the filter medium, the inside filter will not remove anything more than will the outside filter. The only time an inside filter in addition to an outside filter would be required is:
1 - if you add chlorine to treat your freshwater tank, and then drink the chlorine-laden water rather than draining it. GAC will remove chlorine.
2 - If you've allowed fresh water to sit in the tank long enough to go stagnant, in which case you ought to be treating with chlorine, which takes you back to (1) above.
If your outside filter and inside filter used different filter media, I could see a benefit to doubling up filters like that. But as long as both are GAC filters…?
I use an outside filter only (Interstates don't have an under-sink filter to begin with), with the filter at the service pedestal so the water is filtered before it even enters my blue-and-white hose. That way, the municipal water is filtered when I use direct hookup, and so is the water I put in the fresh tank for boondocking.