This is what I got at CWorld last summer, to use washing vehicles at home as well as take with us in the coach:
http://www.portablewatersoftener.com/Water-SoftenersLast fall's 6 week trip tested it, and it worked great. It's a softener, not necessarily a filter. For a filter I could add our previous one to the line, the little blue Camco Tastepure one available in most RV accessory venues. But the On-The-Go allowed pretty good flow, where some filters are restrictive. It made for softer water by removing calcium and other minerals, so showers and clothes and dish washing was nicer. Shop around for best prices, but the CWorld deal was good at the time I was looking, and I didn't have to pay for or wait on shipping.
There are other brands and sizes available, but I really think this was the best for us. It has adequate capacity for our uses, isn't hard to recharge (admittedly I haven't had to do that yet as ours still isn't at capacity), and it really fits nicely upright in a compartment adjoining our water bay where it's convenient; usually though, I just leave it hooked up outside on the ground when camped, and semi-hidden in the corner against the coach where a front streetside slideout obscures it.
-Joel