Yeah, those water softeners on wheels you see at the boat stores cost about $700...a little rich, I think. The benefit is supposed to be that you don't need to mop down after washing down; I've heard mixed reviews, but very few people I know use them, so I'm taking it all with a grain of salt.
In thinking about trying to approximate that with a poor man's rig, I've often wondered if just a regular old whole-house filter canister like you'd get at Homeless Depot would do well enough for the purpose. When you read the literature on those things, they supposedly pull out a lot of the mineral content, depending on how dense a filter media you opt for. I had one in another house, and I can tell you it DID trap a lot of bad-looking stuff after only about a week's use. You know it's time to change when the water flow starts to choke down. It's gotta help, and the good news is that kind of rig only costs about $50 and the filters are a snap to replace when the time comes.