I have one that I've been using with my RV. Maybe check out some of those systems, they can be sub $100.00
I think mine was $75 including shipping. Including a stand, the filter, hoses and a pressure regulator. You don't need all that.
Another linky..
http://www.campingworld.com/shopping/item/culligan-rvf-10-exterior-water-filter/12874