Mine didn't have a filter either, new from the dealer, but I haven't added one. In the past, on our previous coach, I put a filter in-line and found that it drastically restricted flow. You have to watch the gpm flow ratings. Ideally, an under-sink filter could be used for drinking water only, but you'd have to cut a hole through your Corian for the seperate faucet or else restrict the main kitchen faucet's flow all the time. And that wouldn't filter bad taste out of the icemaker supply.
I guess one could use an outside filter to just fill up the onboard tank, and use the pump rather than the direct city connection in parks. We haven't lingered in "bad water country" long enough to empty a tank and be desperate, so when hooking up I taste the water - if it's good enough, we use it direct and fill the tank before leaving. There was once, at the Will Rogers Casino/Race Track/RV Park in Oklahoma, when I wished we hadn't hooked into water; the taste stayed with us for weeks before I could get it rinsed out with soda and good old Oregon mountain water. That's when I learned to taste first. If we'd had an outside filter then, it certainly would have been a blessing. Another time, I ultimately tracked down a 2 week phantom odd taste to a new plastic fitting with rubber gaskets I'd put between the water hose and the coach. You never know.
The water softener takes out calcium and magnesium, but not chlorine taste or other contaminants. At least with soft water you can get things clean, as calcium interferes with the mechanical sudsing action of soaps and detergents, and leaves a film of calcite behind that can build up, or leave "waterspots" on vehicles and dishware, as you know. On the flip side, I grew up on a farm with hard well water - iron and calcium and magnesium. My dentist complimented Mom on my great teeth. When I was about 11, Dad installed a water softener, and it was all downhill from there for my teeth. But, at age 61, doctors still comment on my strong bones, attributed to well water.
I'd think the best option is a Flo-pur or Travel Soft softener and an outside filter, and use them or not according to the nature of the park's water you are tapping into. When the water is bad and the restrictive filter is needed, fill the tank and use the pump for high-volume use like showers. A high-flow filter would be a great invention.
Just my non-expert 2 cents,
Joel
