Bullheaded- I have whole-house RO in my coach, and use it to wash windshield when we stop, as well as coach exterior (funny how that's necessary where they rain sprinkler water on your coach every night, but not funny ha-ha). Works great. We even fill from a hose in Baja when traveling there.
I have pre-filters (one depth media sediment, and one carbon-block), prior to the RO membrane. As mentioned above, filters (outside of some pretty esoteric ion-exchange stuff that will have limited life span on the quantity we are talking) don't remove dissolved solids, i.e. hardness and other salts from the water passing thru. We run a 0.5 micron sediment filter in Mexico which is good for most bacteria & cysts, and then the RO. Even salty tasting Southern California water comes out tasting fresh & clean, not to mention the easy clean up. Whole house RO isn't cheap, but I decided it was one place I wanted an upgrade. The improvement surprised me. When I traded my 06 coach w/my first RO system in it for the 08 coach, I bought the whole system again. Best thing we've done for the coach, both times. My wife loves it.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project