For anyone interested in running a small 5" low flow (meaning relatively low resistance) filter in their system, I think I have all the parts figured out:
Pentek 158133 Filter housing

The Pentek 158133 is a Clear Filter Housing featuring 1/4" FPT. The 158133 takes 5" filters and is perfect for filtration in your RV or Camper (OR COMPUTER!). Clear makes it easy to see the visualize the flow rate.
The 158133, #5 Clear Filter Housing Specs:
* Dimensions: 7 3/8" x 4 5/8"
* Connections: 1/4" FPT
* Max Temp: 125 F (52C)
* Max Pressure: 125 psi
* Flow Rate: 3 gpm @ 2 psi (based on filter)
There's various filters you can use. The least resistance one is 30 GPH.
Pentek R30 478
Other available filters that you can probably use, but definitely don't recommend using in loop permanently due to restrictiveness. Only on setup/maintenance.
http://www.filtersfast.com/Pentek-CB...er-filters.asp
http://www.filtersfast.com/Pentek-EM...bon-filter.asp
http://www.filtersfast.com/Pentek-C2-water-filters.asp
http://www.filtersfast.com/American-...er-filters.asp
These filters use an NPT 1/4" fitting which is not compatible with the standard G1/4" we use in water cooling. So you'll need two NPT 1/4" male to G1/4" female adapters.
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And of course, you'll need two more G1/4" fittings, be it barbs or compressions to integrate into your loop.