I posted this to a filter question, but it applies here, too.
One of the links covers what filters what.
I use both internal and external filters (Camco
) because the well water at home and at some of the parks we go to have a high iron content so the external one that I use (100micron) removes that for me while the better filter within the trailer (1 micron) does the rest of the job.
However, others just use an external filter. Make sure that it filters what you want, though.
Water Filter Filtration
My trailer's OEM water filter is a Water Pur CCI-10-CLW Clear Water Filter Housing and uses a CCI-10-C filter. Everything in standard 10" length will work in that housing. What you want is a low micron cyst filter with as high a flow (in GPM) as you can get. I've bought the F1 model from this place before... RV Water Filter Store: Standard Filter Canisters for Whole RV